If you found the turtle on a road near a pond or lake, you could easily release it back there right now. Be aware that in certain states it is not legal keep a wild turtle as a pet. Turtles outside get plenty of sunlight and warmth; turtles kept in an aquarium need heat and UVB lamps to simulate that and keep them healthy. Some turtle kits come with everything you would need to set up a new tank. A filtered, heated tank with the appropriate bulbs is what you would need at the very least.