While the vehicle is running and heated up, check the fluid level on your dipstick. If its low add the appropriate amount of correct transmission fluid for your vehicle (owners manual). If that doesn't work, try changing the transmission filter if your vehicle has a lot of miles. It can be kinda messy and you have to have acces to a proper disposal facility of used oil. So depending on your resousces it might be easier to take it to the professionals if the fluid level is good.