Have you checked the level and the color of the transmission fluid, via the transmission dipstick? With any luck, you may just need to add some fluid, but you have to be sure to use the type (Dexron, or ??) specified in the Owner's Manual.
Whether it is low on fluid or not, is the fluid on the dipstick pinkish-reddish in color, or is it brown? Does it have a burned smell? Brown or burned-smelling fluid is an indication of problems that need to be attended to, pronto.
Has the transmission ever been serviced? Whether the mfr's maintenance schedule (it should be in your glove box) calls for it or not, trans fluid should be changed every 30,000 miles, so if you have not had this done, then it is time to have the fluid (and filter) changed.
If it turns out that you need something more than just some fluid or a fluid change, remember to check the terms of your warranty. If memory serves me correctly, your powertrain is covered for 100,000 miles--as long as you can prove that the car has been maintained in accordance with the mfr's maintenance schedule. If you have done what you are supposed to do regarding the car's maintenance, a transmission overhaul or replacement should cost you ZERO dollars.