Over-simplified reply but it will work until you get the hang of it.
Stay in your middle chainring up front. When you run out of gears to make it easier to pedal, shift to your inside (smaller chainring up front). This will most likely make your pedals have no resistance so you will need to shift to a harder gear in the back.
When in the middle chainring and you run out of gears to make it harder to pedal, shift to your outside (largest chainring up front). You may need to shift your back gear to an easier gear.
Move back to your middle chainring up front as soon as possible to prevent crossover - which wears out the chain and your gears sooner.
As we must account for every idle word, so must we account for every idle silence." ~Benjamin Franklin