Also, don't forget you should avoid using the extremes in your gearing. Don't use the combination of the big chainring (front) and the largest rear cog (i.e. the one closest to the wheel--i.e. gear 13), and don't use the small ring and the smallest cog (i.e. gear 6). The idea is that you don't want to have the chain running at too much of an angle--it'll wear out the parts faster. Moreover, there is always some overlap in the gear ratios (so, gear 11 is likely close to the same as gear 13).
Some would say you should also avoid gears 7, 12 (mid front with smallest and largest in back).
I would guess the cables are stretched. You might check on-line at www.sheldonbrown.com/articles.html.
He may have something on how to adjust cables.
Once you get them tightened, and you get out on the road, what I said above will be more relevant.