How should I select front and rear gears?
Having more than one chainring (one of the front gears) expands the total gearing range available to you. Having multiple chainrings gives you a higher high gear for cruising at high speed as well as a lower low gear for spinning up hills.
The difference in the number of teeth between chainrings is much larger than the difference between sprockets in the rear. You can think of chainrings as being for major adjustments and sprockets for minor adjustments. You don't need to change gears in front nearly as often as in back.
The simplest gear changing strategy is to adjust the rear gears as needed and then change the front gears whenever you start to cross-chain. Cross-chaining is using the big chainring with a big sprocket or the small chainring with a small sprocket. It accelerates drivetrain wear and sometimes produces noise and grinding feeling.