my 3rd gear is skipping no matter what the other gear is on, and this really messes me up with the fast downhill stuff, i just got my bike, and instead of going to my LBS, i would like to try to fix it myself, how do i do this? also my 2nd gear at 8 and 9 skips sometimes too, but not every revolution like my 3rd gear.

also, since i just got the bike, is there something i am doing wrong shifting wise? can somebody direct me to an article or give me some advice as to when to shift, just in case im doing it wrong?