what is wrong with my cavalier?

i have a 1998 cavalier 4cyl. auto and when i get on the gas it will not gain any rpms. and i can floor it and it does nothing. I was told it is a sensor probably but am asking all you smart people for conformation
my cavalier slowly creeps up to speed, i can floor it and it will not jump at all it just slowly takes its time. it will barely pull a hill and my gas millage sucks. i was told its a sensor.

No RPMs? Have you tried turning the key to "start?"