Can someone explain gear ratios on a 21 speed bicycle to me?

I was wondering if someone can explain, in simple terms, which gear I should be in if I wanted to go from the easiest to peddle to the hardest?
I know you need to know the number of teeth on each sprocket (which I assume determines the diameter) and they are as follows (as best as I can tell) Rear sprockets: 13, 16, 18, 21, 23, 25, 27 Front sprockets 28, 38, 48.