When you first begin playing, try out a bunch of different ones to see which ones you like the most. 

When preparing for competition, focus on having 1 main and 1 backup (in case one isn't unlocked).

Start watching videos to master the moves of your main character. 

Instead of jumping right into Smash, click on Games & More and enter the Training mode. 

This allows you to practice your moves on a passive computer character so you can master your main character.