What is the common food that is OK to be given to turtles?
It depends on the species. The natural habitat of the each species determines its diet as well as other aspects of its care. What species do you want to know about?

There are also omnivorous turtles whose diet can include leafy vegetables, ordinary vegetables (wax beans, green beans, beets, sweet potatoes, etc), flowers, and fruits.

You should never give a turtle milk products, processed and canned foods, and poisonous plants.