First of all it depends on which species of turtle you are feeding.
Map turtles prefer eating meat. A river c****r turtles diet comprises primarily of plants. The Malayan Box Turtle’s feed should include lots of plants with only a small amount of meat. the American Box Turtle is an omnivore which eats just about everything, and the Chinese 3-Striped Box Turtle’s diet comprises mostly of meat. Then there is the Red-Eared Slider, which begins by eating worms and bugs, but turns into an omnivore as it matures.
Foods Omnivorous Turtles Eat

Protein - cooked fish,; boiled eggs; cooked turkey or chicken; mealworms; waxworms; slugs; shrimp; snails; crickets; earthworms; and silkworms. Lean beef, dog food or cat food can be given occasionally. Avoid using raw meats due to the danger of contamination and fatty meats like hamburger should be avoided.

Leafy Vegetables - tufted vetch; sow thistle; grape leaves; fig leaves; endive; red clover; carrot tops; plantain weed; chickweed; dandelion; lettuce, both green and red; turnip greens; collards; and aquatic plants like water lettuce, duckweed, and so on.

Vegetables - corn; wax beans; green beans; beets; sweet potatoes; carrots; okra; green peas; and squashes.

Flowers - rose; dandelion; chives; pansies; petunia; lilies; carnations; hibiscus; hyssop; borage; nasturtium; and geraniums.

Fruits - pumpkin; kiwi; mango; apple; strawberry; banana; tomato; blackberries; cantaloupe; blueberries; grapes; figs; and citrus fruits.

There is also commercially made food available for turtles as well as supplements.

Foods that Turtles Should Not Eat
Milk products cannot be digested by reptiles because they do not have the enzymes required for breaking down lactose, hence dairy products like yogurt and cheese should not be fed to turtles.
Processed foods and canned foods are usually high in preservatives and salt, the effects of which are not known fully. It is advisable not to feed your turtles these foods, or use limited amounts, if you must.
Even though turtles may eat certain poisonous plants in their natural habitat, the specifics of this feeding habit are not well known, hence, to be on the safe side, it is advisable not to have poisonous plants where you keep them, such as avocado plants, rhubarb plants, and poison ivy.

I hope this information helps.

The wild ones mainly eat jellyfish, moss and weeds in the ocean and small crustaceans.