Rear shifter trouble

Could be a few things..

Did you crash at all, laying the bike to the right? You may have bent your derailleur and/or derailleur hanger, which can cause irregular derailleur behavior. Depending on the severity if this is the case, you may or may not need to replace the hanger or derailleur.

How old is your bike, and/or how long as it been since you've had it serviced? If it's relatively new, or it hasn't been adjusted in a while, you may just need to have everything dialed in. The cable+housing settles in on new bikes, which can throw off the adjustment within a few days. If it's not new, and you haven't had it adjusted in a while, it could simply be the sum of many things... friction in the cable housing, misalignment of the rear derailleur, chain stretch, lack of chain lube, loose freehub body, loose QR skewer etc.

Frankly, it's really hard to diagnose such a thing over the internet... you've got to have it there in front of your. If you don't feel comfortable adjusting things on you own (and if you don't know what you're doing, you're liable to make it worse), you should take it into the shop where you bought it- if they're any kind of decent bike shop, they'll do it for you for free. If you got it used, or online, they may charge you a small fee.

It must be a cable tension problem. Try shifting to the highest (smallest) cog, then adjust cable tension. It could also be a bent hanger.