Rats are omnivorous opportunistic feeders, and given the choice will select high-sugar, high-fat foods, predisposing them to obesity. Ideally a high quality rodent pellet should be fed, with small amounts of fruit, vegetables or household scraps being fed as a supplement.
Rats will happily forage for food if it is scattered for them, rather than being presented in a bowl. This is a natural behaviour and will help to keep them occupied and active.
Water should be provided ad libitum, and replaced at least daily.