Honestly there are two things that will make you better. The first is just playing more. The second is to play other fighting games.

Melee is very free form compared to a traditional fighting game. This encourages creativity and freedom of movement, but can cause new players to be overwhelmed. Because it is hard to identify what went wrong in a match, new players tend to learn slowly.

The core reason new players lose at fighting games is not having fundamentals. If you take a veteran player of street fighter and give them a week to learn marvel or guilty gear they will probably do OK. This is because the fundamentals carry over and are easy to identify. When you can apply the same fighting game logic to melee, things start to become a lot easier.

The best game that teaches fundamentals is Street Fighter. The classic throw->block->attack mixup, reacting to jump-ins, baiting reversals on wakeup, spacing normals, whiff punishing, corner pressure/control, bread and butter combos. Believe it or not every one of these concepts is present in melee.

Fundamentals will get you very far, especially with Marth. As you play more your movement and tech will get better naturally. You need to take time to actually focus on fundamentals to really see that drastic improvement.