sounds like its not clamped down tight enough. make sure the cage of the derailleur is 1-3 mm above the largest chainring (you should be able to fit a penny in there). also make sure it is parallel to the largest chainring and clamp it down tight.
it also sounds like you have a problem with chain tension. loosen the cable clamp on the derailleur and then loosen the barrel adjuster on the shifter. then pull the cable as tight as you can and tighten the cable clamp. then you can tighten the cable with the barrel adjuster.
also check your limit screws