Remember that turtles can and do bite. Also watch out for claws. I know it may be interesting to see your turtle tuck its head and limbs in its shell, but please do not intentionally provoke this behavior. Keep your other hand on top of the shell, holding securely, so that it will not fall and if it tries to escape, you have a firm grip. This is just plain common sense.
Turtles live a long time.
Before you get a turtle you have to think about the long term. Ask yourself if you will be able to care for a turtle in the long run. Consider the cost. As a turtle gets bigger, it gets more expensive. Larger quantities of food and a larger tank are just the beginning.