Another way to think about is: the gear closest to the bike is easiest. In front, it's the small chain ring. In back, it's the largest cog.

The gearing on my (road) bike is different, so I'm not sure if this translates to your bike, but for most hills I stay in the middle chain ring in front and shift to easier larger cogs in back. After shifting to the largest cog, if the hill is steep I will shift down to the small chain ring in front. At the top of the hill, I shift back up to the middle chain ring, and then shift to smaller cogs in back as needed for the terrain.

Practice shifting while riding someplace flat and with no traffic, so you can get used to shifting between easier and harder gears and you can safely look down to see which way the chain moved after you shift.