The 2 most important things to picking a main is.
Being good with that character. For example you might find a character fun, that´s great! But that character is horrible and you lose all the time with him/her.
Finding a character fun. That is one of the most important aspect of smash. You can be good with a character, But how fun is it if the character is boring?
It also helps if you find the character "cool".
All in all, you should play 1 or 2 matches with every character. So you can se how they feel. Then you round it down to 5 characters and play with them. Then you should find your main.
I have been a puff main ever since 64. I was 18 years old when i first played it. So i have puffed all these years.
Jigglypuff is the reason why i have won so many tournaments.
So she has a very special place in my heart.
When you have picked your main, then that character will stick with you. In the end, that character will become an important part of your life. Just like Jigglypuff is important to me.