2000 Chevy Cavalier High RPM at startup and idle

My 2000 Chevy Cavalier is reaching 2000-3000 rpm's at startup and reaching up to 1500 at idle. I took it to a Chevy dealer and the computer said that a transmission sensor was bad so they replaced that. However, the problem did not go away. I have tried to do some research on what could cause this and have ran across low battery power, spark plugs, and Mass Air Flow sensor. I'm hesitant to take it back to the dealer and am just trying to find out what it could be. It also has some difficulty when switching gears and has a slight kick. I would like to try and fix this problem and sell the vehicle but do not want to put a lot of money into a $2,000-$3,000 car.

Greg