Bicycle Mechanics - My first mountain bike problem

I'm just wondering. I was cycling on my brand new Carrera Vengeance and the gears seemed to shift themselves when I was riding up a hill which isn't too steep. Is that normal for a mountain bike? Sorry for the stupid question but I'd rather be acknowledged something is up with my bike rather than have even bigger problems with it later on. Thanks in advance. :)

Chances are the cable tension on the shifter cables isn't quite right. If you bought it from a shop, this generally means it's time for its 30-day check up, which every shop I've worked at does for free. The cables are braided steel and stretch, and the housing tends to settle in which effectively changes the "length" of the cables. Bring it back to them and you should be all set.

Hmm. They did the servicing before letting me go with the bike and they said everything is fine. Looks like they didn't pay much attention to it then. Do you think it can get worse over the time if I won't fix it?

It usually won't get worse (but it can), but it's not going to get better, either. Take it back in and have 'em do a quick tune up, as said most shops do one or two for free, sometimes a whole year of free quick tune-ups.

Parts on a new bike have to settle in a bit. Most of this occurs easily by the time you put 100 miles on the bike, then you fine tune again and you should be good to go.

Yeah, it will only get worse. What you likely need to do is adjust the cable tension at the rear derailleur. That can be accomplished by turning the barrel where the cable goes into the back of the derailleur.

Screw the barrel in to lower the cable tension, and unscrew the barrel (screw it out) to increase the tension. You'll likely have to do the latter, since the cable is stretching. Turn it 1/4 of a turn and try it out until it feels right.

It also could be that you're derailleur hanger, where the derailleur attaches to the frame, is bent. It's a replaceable part on most bikes just for that reason. This would be easier for the bike shop to miss, so if you can't get the derailleur dialed in, that's you next suspect.

Alright, thanks guys! :D