Like people, rats are omnivores. Also like people they will happily consume more junk food than is good for them given the chance. If you want your rats to live long active lives and have the best possible health, choose their diet carefully.
If you don't want to use commercial rat foot, your rats will do best on the sorts of foods that are also good for humans like wholegrain rice, fruits, vegetables, grains and wholemeal bread, supplemented by a small amount of meat. A daily food mix comprised of a bit of all of the above will be a good diet. Adult rats should get 15-20g of food every day. Note that even if you are vegetarian, your rats aren't and you should still give them some animal protein. As long as the rats are getting a healthy diet every day, it's fine to give them an occasional unhealthy treat such as a small piece of chocolate
Your rats can certainly do well on a well balanced home made diet, but if you prefer, it is possible to use commercial food mixes designed for rats available through pet shops. However if the food is made up of a mix, you will have to make sure that the rats are eating everything and not just their favourite bits, otherwise they aren't getting a nutritionally complete diet. Don't keep topping their food up if they are in the habit of leaving the less preferred parts. Nutritionally complete pellets get round this problem and may be the easiest food base for your rats, since you can be confident that they are getting everything they need. However imagine how bored you would be if you had the same thing to eat every day for your entire life. If you use pellets make sure you provide some more interesting (but healthy) fresh food treats like fruit and vegetables that they can get excited about.
Rats should always have fresh water available. The easiest way to provide it is a gravity bottle (or two) which can be attached to a cage. You should change the water every day and clean the bottle once a week. Your rats would probably prefer it if you gave them a bowl of water to drink from. Not least because they will happily wash themselves in the same water. As a treat for the rats you might like to do this from time to time, especially in hot weather. The downside of giving them a bowl of water is that you will have to clean the cage more often. And if the bowl isn't heavy enough you might find the contents tipped over the cage.