think ahead. what you're probably doing (forgive me if i'm wrong) is that you hit an incline, think "damn, this is harder than it looked!" and shift into an easier gear. don't shift while the bike is under load. instead, anticipate the difficulty of the hill coming up.
shift into an "easier" gear before you get to the hill. even shift down more than one, if you want - no worries. this will increase your cadence, but lower the load on each push of the pedal. if you find you shifted down too much, you can shift up. you would have to have a fast cadence to do this, though - we're not leg pressing here at 60 rpm.
you'll figure it out as you ride more.
now, you may question what is a good cadence, since you're a "noob." Well, i'm sure different people here will give you different answers, but for mountain biking i say nothing less than 80rpm. i aim for 90 - 110 when i ride (roads or trails). usually after a hill, i shift into a lower gear, increase my cadence, and give my legs a "break," so to speak.