Step 1: Pick a character
The first step to learning how to play well at Super Smash Bros is to pick the character you want to get good with! This character can be anyone on the roster, it doesn't matter who. However, depending on why you want to learn, I can make some reccomendations.
If you're playing for fun, I'd go with a character that you just like, or with a character whose play style matches your own.
If you plan on winning big at tournaments, I'd do some researching into tier lists and go with someone near the top.
If you want a challenge, or just want to try and evolve the metagame, go with someone near the bottom of the tier list!
The guy on the left, Little Mac, is tiny, meaning he's quick, but easier to send flying. The guy on the right, Bowser, is bigger, meaning he's harder to knock away, but a bigger target.