I'm a total newbie here myself, but DON'T PANIC! You can do this.

Test the water yourself before you do anything. Those guys at the pool store are like first cousins to slimy used car salesmen (apologies to any honest used car salesmen on the board).

Do you have the bleachcalc downloaded to your computer? It's a great tool..look for it in the signature line of mwsmith's posts (think I have that right).

You're going to want to bring up the pH and alk..the bleachcalc has a place under "more tests" where you can figure out how much you need to add based on the gallons in your pool. You also need to increase your free chlorine a bit, but I think I've read here that if you get your pH into the right range your chlorine will be more effective.

Most importantly...wait for someone who actually knows what s/he's doing for real advice. My real advice ended at DON'T PANIC.