Talk to the LBS guys and have them show you how to adjust the cable tension. Or go look at the Sheldon Brown site or Park Tools.

Just so you know, 99.999% of all shifting problems are due to cable issues. The other 0.0009% is due to other issues. The remaining 0.0001% might be due to limit screws. If the bike shifted okay yesterday and shifts terrible today, it's a cable problem. In fact you could probably save some weight on the bike by taking a dremel to the limit screws and cutting off the heads...that's how much you'll need to adjust them (Don't do this!!!!! But you could)

Messing with the limit screws on a bike should be the next to the last thing you do. After you've exhausted every other remedy, including consulting with a voodoo priestess to take off the curse, and just before you are getting out the hacksaw to cut the bike into scrap, you can try adjusting the limit screws.

Luckily, you didn't do any lasting damage but misadjusting the limit screws can result in real damage. Adjusting the high limit screw on the front derailer is the most benign. Not much damage can be done with it. Misadjusting the low limit on the FD could, at the worst, result in chain suck and a damaged frame. Misadjusting the high limit on the rear could result in some frame damage. The worst you can do is to misadjust the low limit on the rear derailer. This could result in shifting the chain into the wheel and can tear the wheel completely up, along with ripping the derailer off