Turtles are not the easiest pets to care for. They need very specific care that includes a very large enclosure (most people end up building there own because the ones they sell at pet stores are not very good) , special UV lighting, exact temperatures and specially prepared diets. The turtles need these things to survive in captivity. Most pet turtles die because the owners don't know how to or cannot care for them properly and give them the things they need. You need to do some research on the type of turtle you have and it's care requirements.Here is a good place to start.
http://www.austinsturtlepage.comThere you can find care sheets for all types of turtles sold as pets. Get the turtle everything it needs right away. It might be expensive to set everything up but otherwise your turtle will die. If you decide the turtle is not for you then call a local reptile rescue center and they will take it. You could bring it back to the pet store but they would just sell it to someone else that couldn't care for it and it would die anyway. Good luck