So, if you wanna get good at melee you're going to need a few things. We're not going to assume that you are starting from completely nothing.
Melee is a hard game, and Link is arguably the most difficult (borderline unviable) character to get good with. You're playing hard mode (melee) on top of hard mode (difficult character) You're going to need not only nerves of steel, but every tool available.
You will need General Melee knowledge, General knowledge on how competitive games work, and how you should think about them, as well as specific knowledge on link as a character.
- #3 Improvement
Even just these two guides are a LOT of information, don't expect to absorb it all at once. Just work on one or two things at a time from it, and incorperate it into your game. You're not going to get good at melee in a year. You're not going to get good in even two years. Some prodigy's have done it in three, but most of us are just normal people and it's more of about having fun learning instead of hyper focusing on simply improving for the sake of it. Enjoy the time you have to spend on this 20 year old party game, because at the end of the day, that's all it was made to be.
One of the first things you want to be decent at, when playing Link (or really anyone in melee) is Movement. Please practice it, and practice it often. Links do not spend enough time working on this and it shows. When you can't get to ledge quickly, or do all your options from ledge fluently, it really hurts your game.
The second thing is learning your combo B&Bs, and flowcharts. These can be found on the character pages.