So if you find a wild turtle in its natural habitat, it's best to just leave it there. There are actually laws protecting most of our native turtle and tortoise species in this country. and it's actually illegal for you to take that turtle home and attempt to turn it into a pet. They also don't make particularly good pets. They're not used to captivity, they're afraid of people, and they often don't eat well on our captive diets.
If you find a turtle that's trying to cross the road, and it seems otherwise healthy, you can help that turtle across the road. Try and note what direction it's going and try to help it to the side that it's trying to go to. Never drive that turtle away from the area where you found it. Most of them have home ranges or territories where they have shelter and food. And that turtle is probably just seeking out some food or a nesting area. And if you move it, it could actually affect its survival.
If you find a turtle that's been injured, then the best thing to do would be to note exactly where you found it so if it can be released after medical treatment, we can return it to the same area. Then bring that turtle to a veterinarian or to a wildlife rehabilitator. So most states have listings of their licensed wildlife rehabilitators. And if not, you can always contact a local veterinarian who will hopefully be able to give you that information.