I'd suggest finding another bike shop (or bike coop) to help you with your problem.
At the very least, try to be more descriptive. "I try to push on the lever but it doesn't do anything" doesn't give us the info we need to try and help you. Do you mean the lever doesn't move? Or the lever moves, but the cable on the other end doesn't move? Or the cable moves but the RD doesn't move? or ????
Start with the Park Tool site and lean the basics so you can better communicate to us. And good pictures are often helpful too.
And of course Sheldon Brown has a ton of info too; learn the lingo so you can clearly communicate the situation and you will be rewarded with plenty of good advice.
But you will be best served by getting somebody local to look at it. If the shop you went too gave you poor service, find another shop or coop.