Violette,
it's hard to tell from your description if there is something wrong or not. When you say the shifting is "terrible", is that because it's loud, or for some other reason? This loudness -- it is just the "clunk" when the chain moves to a new ring, or is it rattling? Are you missing shifts?

I think you should probably take it in during a slowish time and have the mechanic put it up on a stand. You can explain what's bothering you, and s/he can both look to see if there is something wrong, and give you a better idea of what you *should* expect.

Good luck!