you didn't say what kind so Ill assume you don't know 😉
if the turtle is a box turtle lower shell folds up to cover face): you can feed it earth worms or green leaf vegetables or box turtle food at the local pet shop (earth worms are the best). keep it in a box or tank temporarily with a jar of water.
if the turtle has thick armor on its limbs and the upper and lower shell is beige and scuffed up (maybe a brownish and yellowish hue visible) it is possible a desert tortoise. It can eat grass, dandelions, hibiscus flowers. Just let it go in your backyard and it can feed itself. it may get lost if you leave it too long because it will dig a hole. if you have any vehicles, always look under them because the turtle will seek shelter under them. give it water once a day by putting some water in a hole with the water hose.
if the turtle is green and has webbed limbs and bright orange or yellow stripes on its neck with a dark green shell, it could be a water turtle. Keep it in tank of fresh water (changed daily) with rocks so it can rest on. Feed it earthworms and goldfish.
if you keep the turtles indoor, put an incandescent light over them during the day so that they can keep warm.
any more questions, please post