Find a friend to help you (preferably somebody who rides, but doesn't have to be). Hold the rear wheel off the ground and have your friend pedal with his hand. (Gears won't work unless the pedals are moving forward.) At the same time, fool around with the shifters and watch what happens to the chain. Pretty soon you will have some understanding of how your gears work.
Then hop on the bike and practice riding and shifting in a safe and non-stressful location. You'll get the hang of it pretty quick.
Just keep telling yourself, "If a 12 year old can do this, I can too!"