Originally Posted by Retro Grouch
Why that's easy.
1. Before you ever start out, move the shifter on your left handlebar until the chain is on the middle front sprocket. Leave it there most of the time.
2. Now you can concentrate on just the right shifter. If your legs hurt, shift into an easier gear in back. If it seems too easy, shift into a harder gear in back. Most of the time, that's all that you need to know.
3. If you see yourself approaching a big hill, shift the left shifter into the smallest sprocket in front just as you get to the base of the hill.
4. Two or three times a year you will get a tail wind or want to ride downhill real fast. When that happens, shift your left shifter so that the chain goes into the biggest sprocket in front.
I thought I might take a crack at this, but hats off to Retro Grouch. He's got it right - for the front sprockets, the middle is your gold, the others you hardly ever use. The simplest and best answer was the first. There's no point trying to muddle it up even more with my own garbled explanation.
He's used the KISS rule quite well, take his advice (sorry, just assuming it's a guy).
And don't get too bamboozled by the other posts - I've been riding for years and understand it all, and even I don't get some of their explanations.
Enjoy your riding!