Play around with all the characters in the roster. Eventually find a character that you are good at and make that character your main. Learn that character's moveset and don't keep switching mains every week because that can lead to constant relearning of characters and slow improvement.
Know other character's movesets so that you know what moves to deal with when facing your opponent.
Watch videos of pro players and participate in tournaments. Losing and learning is better than winning and not learning anything.
Stop making bad habits and review your mistakes by saving your match replays.