The fundamentals of competitive gaming are really a mindset more than anything else. People who are discouraged from playing by a loss during friendlies or practice are simply not ready to play competitively, because they are playing against themselves and not their opponent.
Practice
This is obviously the most important part of competitive gaming, but also the most misunderstood. Playing a lot, is not practice. Practice is reliant on the idea that you have a concept in mind and you are dedicated to applying your time to working on this one thing over and over until you are comfortable with it. Practice partners are huge, having a good competent partner will help you learn faster than anything else. Set a 99 stock match, and if for example you want to practice ledgegame and recovering, run off the edge every single stock until you are surviving longer and taking advantage of being off stage. Your partner will be simultaneously practising his edgeguard, and will have input as to what bad habits you have/are developing, and what else you can try. Switch roles, and your brain will immediately provide you with new ideas as you get a rounded experience. Focus on loose concepts and aspects of the game, and you will experiment with new and original styles with varying degrees of success. This is imperative to finding your own strengths and not relying on metagame/board discussion for what is "right or wrong" as whatever you are most effective with will be best for you. Practice is where you become a strong player, with a unique style that will give you the edge in competitive play.
Attitude
You are playing a game, and the best way to get better at it is to remove your ego. The way you want the game to be, is not how the game is. If you are damned and determined to make your Luigi the best Luigi in the world, and can't seem to beat the majority of people you play, whatever it is you are doing is not working. This does not mean you or Luigi are bad, or flawed, but simply that you are stuck on the fact that you don't get to win doing whatever you want. This is hard for many people to get over. Learn how the game works and how the mechanics operate. Practice these basics, and ignore deaths, ignore kills, ignore who beats who or which of your friends is winning more. You are playing to get better, and to get better you must only be thinking about getting better. If you want to learn how to shuffle, keep shuffling, and if you aren't getting it, keep shuffling. If you don't think shuffling is going to help you be a better Falco player, stop thinking that, and keep shuffling. You will get it, and you will be able to do it more consistently. If you stop practising because you are not succeeding, you will never succeed. You will never be an excellent painter if you give up after your first few paintings are awful and you don't understand it.
Meta-game
This is all too often expected to be a "what beats what" solution to people who are frustrated with losing consistently. X move does not beat Y move, every time. You will benefit from reading what moves are best applied in what situations, but until you spend the hard time practising those situations, you will never know why until you have done it repeatedly to find our what gives you the most success. Watching videos is a great way to see new techniques and combos you didn't know before, but this is not a solution to beating that player, or that character. Always take new ideas from the world of players, but always spend the time experimenting in order to understand why they do what they do. Knowing Sheik's nair nerfs ZSS's laser does not mean that nairing every laser is a good idea. When you are faced with a problem, read and learn about what else you can try, but never assume there is a move you can do that means you don't have to practise the situation over and over again.
Tournaments
This is where your practice (and you may have noticed it's all I ever talk about) comes into execution. You are no longer playing your friends, or your practice buddies, you are now playing where losses count. When you play for extended periods of time, you develop hard habits and constant routines that will no longer be applicable, and you will now be relying on your core concepts and experience to get you through the match. You are now playing your opponent, which means in order to truly be effective, you must be reacting to what they are doing. Reaction is where your muscle memory and regimented exercises take over and when your opponent acts, your reaction reflects the amount of time you spent practising. If you are surprised, and confused by what your opponent is doing, you must adapt quickly, and fluidly. To play smart, means that you are not committing yourself to your own actions, and are waiting for your opponent to give you the opportunity to defeat them. Your biggest strength in a blind match is that your opponent doesn't know what you are going to do either, so your practice and comfort with your character is going to shine through when you do the unexpected (read; not the basic meta you read everyone copy and paste on the forums) and force your opponent to react. Do not repeat yourself, and do not keep trying what is not working. To be truly effective, your plan must be no plan. Your plan must change with every circumstance, which means with every circumstance you must have a new plan. That thing you really wanted to try will get you killed if you dont know when to do it, and that will always be determined by your opponent.
Variety
Your first tournament explodes your mind as you will meet a huge number of different playstyles and opponents. You will lose in ways you have never seen before, and find yourself better at things you had no idea were even that good. This will test you in ways you can't possibly predict, which will be the truest tests you face. To have one teacher is to learn half of what he knows, and all of his bad habits. You will need to learn from every opponent you face, and to grow in this regard is to constantly challenge yourself to find new situations to test yourself. Having many practice partners will help you smooth out your bad habits and weaknesses, and really start to help you see the sweeping trends in your play and the core concepts of the game you need to focus on. You will benefit greatly from learning what you do well, and what your opponent does poorly, because they are very different ideas that can be easily mistaken for one another.
Your Mind
You benefit greatly from learning about yourself, and the way you act or react. To know yourself is to know your strongest enemy, and you will never know yourself until you learn to acknowledge your deepest fears and weaknesses. This acceptance will allow you to forgive yourself and others, which means you will be free to transcend the distractions of anger and fear. Conquering your ego and removing yourself from the equation is the biggest obstacle many will face when it comes to getting better at anything. Training your muscles will build strength, but you must train your mind in order to use that strength effectively. The reason we can be defeated by any opponent is that you will always be outnumbered by the second opponent living in your head.