Well obviously the real answer is years of training and hardwork! As for indicating that someone in your lineup is a goalie, that's easy. Go to your lineup by clicking "Manage my skaters". Each user listed in your lineup has a "G" beside their username. Click on the G for the user that should be a goalie.
The user's with a bright red G are goalies, everyone else is a skater. Goalies never have to pay to play.
Here, I made a video to show how to do this!