Lettuce is mainly water. Since it has very little nutritional value, it's best not to feed it to him.
Can you describe your turtle to us, so we can try to determine what kind it is? Or maybe find a picture of a similar turtle online?
First of all, have you researched turtle care and set up a proper enclosure for him? This will need to include a basking spot, a heavy duty filter (turtles produce a lot of waste!), a UVB/UVA light, and a basking light. A heater may also be necessary depending on the type of turtle and were you live. If you aren't ready to properly care for him yet, I would suggest letting him go where you found him. You can always get another once you've researched turtle care and set up a good enviroment.
Many aquatic turtles are carnivorous while young, and will start to eat a more varied diet as they age. An adult is usually omnivorous. This will depend on the species though. Small crays, bloodworms, blackworms, insects, insect larvae, and commercial pellets are good for juvies. Supplements may be a good idea also.
As he grows he will need vegetation in his diet. I'm not sure exactly how old he is now, but you may want to offer aquatic plants, like anacharis. Snails, slugs, tadpoles, & small fish are also eaten my wild turtles.
Let us know how it goes if you decide to keep him. Good luck!