i just got this bike a day ago used from somone the bike was brand new and this is the first mountain bike i have owned i think somthing need ajusted because when i shift the rear gears thay will shift but then reshift either up or down is there somthing i can do or what need to be done
Most likely you need to adjust, or have a LBS adjust, the rear d cable, most likely a quick adjustment to the barrel will be all you need. If the d hanger is bent, that could cause it as well. If the bike has been sitting for some time clean and oil the chain. If you don't have a lot of experience with that sort of stuff, having a LBS do a quick one over would be money well spent. Most good LBS if you ask will show you how to make the minor adjustments to keep it shifting smoothly.