Rats are wonderful pets, but you need to understand their needs before adding them to your home.

Supply Checklist:
--A Wire Cage
--Bedding such as Aspen, Fleece, or Carefresh/Recycled Paper. NEVER use Pine, Cedar, or Clumping Kitty Litter, as they are incredibly toxic to rats.
--A Water Bottle
--A Food Bowl. I prefer to have one for dry food, and a second for wet foods such as fruits and veggies.
--A High Quality Lab Block food.
--An 11.5"-12" Wheel. Wire wheels stay clean longer, but solid wheels can’t catch tails or toes.
--A house or igloo. Rats need a place for privacy, preferably large enough for two rats to cuddle in.
--A Lava Rock Bird Perch or Cinder Block/Brick. I prefer to put these in a high-traffic area of the cage, as it helps to file down sharp nails!
--Wooden Chews and Hanging Bird Toys. Rats are incredibly intelligent and need stimulation, or else they will get bored and/or depressed. Some rats may even chew through plastic cage pans if they don't have any toys.
--A hammock. Rats love to cuddle and snooze in hammocks, the more you have in your cage the better.
--Bleach and Dish Soap. I find these are the best cleaning tools for rat cages. I also use baby wipes between cleanings to freshen up particularly dirty areas.

Behavior
I recommend playing with your rats for 30-45 minutes after first bringing them home, and then giving them time to explore their new cage. Be gentle and patient during this time, as they are surrounded by new sights, smells, and sounds, and will need time to adjust and become familiar with you. Try to have at least an hour with rats outside the cage every day, even if it's 30 minutes in the morning and 30 minutes at night. Rats also love routine and pick up on it very quickly, so sticking to a schedule can be beneficial for both rats and humans. Rats also require companionship, and therefore must be kept in same-sex (or spayed/neutered) groups of at least two rats. They will become depressed, anxious, and ill if left alone, unless they show aggression issues with all other rats. Even then, a lone rat will need several hours a day to play with their owner, but it will not be the same as having another companion of their own species. Personally, I prefer larger groups as the rats form unique relationships, it’s incredible and adorable to watch!

Daily Care
Make sure you provide your rats with fresh food and water on a daily basis to keep them happy and healthy. I will usually provide fresh water right when I wake up and right before bed, but this may differ depending on how much your rats drink (however, the water bottle should be refilled at least once a day). I refill the food bowl with lab blocks in the morning, and in the afternoon or evening I provide fruits, vegetables, and other healthy snacks. I will do this in the cage or during their free-range time, depending on how messy the snack is.

Diet
High-quality lab/rat blocks are a staple in your rats’ diet, and should be available to them at all times. They should also get some fruits and veggies daily, such as peas, carrots, blueberries, etc. Some owners prefer to make their own mixes instead of lab blocks, but I find that rats will pick out their favorite pieces and ignore the rest, meaning that they won’t get the full nutritional benefits of their food. Some of the best lab blocks are:

Oxbow 

--Oxbow Essentials - Mouse & Young Rat (under 6 months). However, it is not my prefered young rat food, as corn is listed as the first ingredient.
--Oxbow Essentials Regal Rat Adult Rat Food (6 months and older). My absolute favorite rat food. It is a great formula, and can be fed to young or pregnant/nursing rats as long as you feed them extra protein (Scrambled eggs, nuts, etc.).

Mazuri

--Mazuri Rat & Mouse Diet - It has really high protein, and should ONLY be used for pregnant/nursing and growing rats.
--Rodent Breeder 6F - For rats 6 months and older.

Harlan Teklad Lab Blocks

--Teklad Global 2014 - This is intended for rats 8 months and older.
--Teklad Global 2018 (Native Earth) - Made for pregnant, nursing, and growing rats (Younger than 8 months).

Cleaning
I find that the Rat Whisperer had a great outline for cage cleaning, which I personally follow for my cages.

Daily:
--Wipe bars, levels, and ramps with baby wipes.
--Check if hammocks are soiled and clean as needed.
--Remove as much heavily-soiled bedding or poop as possible.

Weekly:
--Remove all old bedding.
--Wash out entire cage with dish soap and warm water, or wipe with baby wipes.
--Wash all hammocks or other fabric pieces.

Monthly:
--Remove all old bedding.
--Hose down bars, levels, ramps, tray, etc.
--Disinfect all plastic, metal, and ceramic accessories with a diluted bleach mixture (About a cap full of bleach to 32oz of water).
--Use bleach mixture to disinfect bars, ramps, levels, tray, etc.
--Scrub down all cage parts and accessories with dish soap to remove any bleach residue.
--Dry everything off, or allow pieces to air dry.

Cages/Housing
Rats need solid-bottom, wire cages with ½-1 inch bar spacing, smaller if possible. Cages for female or baby rats should not exceed ½ inch bar spacing, as they can squeeze their way out! A pair of rats needs a minimum cage size of 2' X 2' X 2', but it is best to get the largest cage you can afford. Cages must be kept indoors, away from drafts, direct sunlight, or extreme temperatures. Ratty Corner has an incredibly helpful cage calculator to see if your cage is big enough, and it’s super simple to use!

Vet Care/Watching for Illness
You should have your rat checked by a veterinarian once a year, but you should also know how to look for signs of illness so you can take them to the vet as soon as possible. Keep an eye out for sneezing (respiratory infections are often fatal in rats if left untreated!), weight loss, difficulty breathing, lethargy, diarrhea, dull eyes, open wounds, or lice/mites (unexplained or excessive scabs, or small moving particles on a rat’s fur), and do not hesitate to see a vet if you think something might be wrong!

Dangerous Foods for Rats
There is a more comprehensive list of the foods rats can and cannot have on the Rat Forum here, but I will be focusing the foods they should NEVER eat. List copied and pasted from Rat Forum.

Fruit:
Apple seeds - Apple seeds contain a small amount of cyanide which could be harmful to your rat. (Original post by Nightfallspy)
Green bananas - Inhibit starch digestion
Mango - Contains d-limonene which can cause kidney cancer in male rats (Thank you Darksong17!)
Oranges - The white part on the peelings and outer layer of a peeled orange is harmful to male rats. It seems to be ok for females, but I just stay away from citrus fruits in general.
Orange juice - Contains d-limonene which can cause kidney cancer in male rats

Vegetable:
Raw brussel sprouts - Fine after cooking, though
Green potato skin and eyes - I will not eat them and I will not eat green potato skin and eyes
Raw artichokes - Inhibit protein digestion
Raw onion - Can lead to anemia and an upset stomach
Raw red cabbage - Fine after cooking
Raw sweet potato - Once again, fine after cooking
Rhubarb - High levels of oxalates which bind up calcium

Miscellaneous:
Blue cheese - Contains a toxic mold
Cuddle Bones (for Birds) - The general consensus was that these have far too much calcium to be healthy for your rat, steer clear. (Original post by sneakers8)
Licorice - Contains a suspected neurotoxin
Poppyseeds - One member's rat got into these and then began to act very lethargic. The rat seemed better for a while, but unfortunately the situation deteriorated and the rat had to be put to sleep. (Original post by rainbowstar)
Rat poison - Believe it or not, this is just not good for rats
Raw bulk tofu - Packaged tofu is ok for eating
*Raw* dry beans or peanuts - Peanuts you buy in a store are almost always roasted so they are fine for rats. Beans will need to be cooked first before giving to rats. If you're growing your own peanuts then I would not serve those to your rats.
Rawhide - Difficult to digest (Original post by hydra)
Sweet feed (For horses) - There's just too much corn and molasses in this. (Original post by Krickette)
Wild insects - Well I guess these are ok cooked? (Mealworms, however, are safe and a good source of protein!)

Undecided (Best to avoid for now):
Mint/Menthol/Candy Canes - It is currently unknown as to whether or not this may be harmful to your rat. Best to avoid for now. (Original post by zimmyzam)
Peppers - Though the non-spicy portion of the pepper is fine, you may want to avoid these as you'll never know if some of the spiciness got on the non-spicy portion, which could be very upsetting to a ratty. (Original post by glindella)

What your rats should eat in moderation or with caution:
All of the following should be given in moderation. Some can be very dangerous to your rat if given too much. Personally, I would recommend just not giving your rat any of these (With the exception of chocolate, which is fine in small amounts).

Avocados - Very fatty. Also, the pit, rind, skin, and leaves of the avocado are toxic. Avoid any part of the avocado that even touches those parts. Good for sick rats needing to gain weight though
Bread - Bread can expand in a rat's throat, causing them to choke. If giving your rat bread, toast it first or give them very small amounts at a time.
Carbonated drinks - Though I doubt your rat will explode, it could give them a stomach ache as they have a hard time getting rid of the carbonation.
Chocolate - Supposed to help with minor respiratory distress. Too much can be problematic. Avoid milk chocolate but rather go for dark or bittersweet chocolate as milk chocolate contains too much fat and some rats are lactose intolerant (Thanks Mana!)
Dried corn - Contains fungal contaminates which could lead to liver cancer (Fresh corn is ok)
Oranges - The white part on the peelings and outer layer of a peeled orange is harmful to male rats. It seems to be ok for females, but I just stay away from citrus fruits in general.
Peanut butter - Could cause your rat to choke