I've only started to play the game for less than a month, and already I have a bunch of "mains". It's pretty much just characters I really enjoy playing. I first started off with Ganon, and he will always hold a dear place in my heart, as well as the character I would probably main at a moment's notice. However, I can also play a pretty sick CF, I really like Squirtle's playstyle, Lucario just looks like a beast, Wolf is the easiest of the Spacies for me to play, Diddy has great banana game, and I really like Wario's Up-Air. I also used to play a bit of Roy and Marth, before realizing I really suck at them. However, I think I'm slowly starting to have three characters that I would probably play in a competitive scene, which would be Ganon, Captain Falcon, and Squirtle. After playing a multitude of characters, I just came to realize I liked characters that did a crap ton of damage while using that big "surprise factor" to quickly outplay your opponent. For example, Ganon looks really slow and clunky, yet he has a great aerial game and has a lot of kill potential. Falcon's main kill move is his knee, and it's so crucial that a lot of mindgames have to be played in order to land it. Squirtle looks hecka small and weak, but his aerials are amazing and his Up-Smash is god. By quickly understanding what I like to play, I got to easily weed out characters that didn't seem so interesting, such as Samus, ZSS, and Ice Climbers.
Essentially, what I'm trying to say, is that eventually, you will find a playstyle that you really enjoy over all of them, and that will allow you to select certain characters that you know you will enjoy. Because I knew I really liked Ganondorf, when I first played Falcon, I already guessed that I would like him as well.