The fact that you know how to use the internet means that you can find out the answers yourself. One way to start is to look up the websites of airlines that you may fly.

Essentially, you receive credit (in miles) in a particular airline's program for each trip you make. When you receive enough miles, you can try to get a award ticket (if the airline says it has seats for you).

Flyertalk has posts about other ways to get miles. These include MR (mileage runs, which are pointless trips that don't costs too much but earn miles), bonus miles opportunities, car rentals, credit cards that get miles.

Don't let your miles expires. Some airlines miles expire after 3 years of account inactivity.

Consider the small airlines programs like Midwest, Frontier, Alaska (actually a fairly large airline, though smaller than the likes of American, Delta, etc).