The only way to learn how to tech, Marth trips on Temple.
Basically, you should approach the universal systems first, DI, wavedashing/spacing tools, and ledgegame/recovery. Learning how the different aspects of the game works will give you the understanding you need to pick a character and playstyle.
Picking a character is going to involve playing many of them, and watching videos, to see what looks appealing to your specific style. Some people like technically involved characters that require deep skill and training (Fox, Falco, Falcon.) Some enjoy all-around characters who are more rounded in most areas, where their specific moves and how you use them will be what sets you apart (Pikachu, Toon Link.) Others like slower characters that require finesse, mindgames and tricky usage of a few tools (Bowser, Ganon,) and some people play Sonic.
Once you have an idea of where you want to start (being aware of your own strengths as a gamer really helps with this) you will have an idea of where to start with the advanced techs like wavedashing and shuffling. If you are starting the game from ground zero, you dont want to jump in and start trying to master high end techniques in hopes that it will make you better at the game, you really need to work into it. While they become mainstays in competitive play, they are in no way "the basics" like you may see them referred to here on the boards.
So, here's my list of basics of getting into a new game:
- Find out how you want to play - Research the game by watching videos of different characters, if you look through the brawl and melee character forums, there is a wealth of information about every character and plenty of videos provided by some of that character's top players in a variety of matchups. Until you can see someone really good doing what you want to do, you wont really know what you're aspiring to, which leads to frustration and boredom when you dont have a personal set of goals.
- Get involved - Unless you happen to live on a farm 150 miles out of nowhere, Iowa, well, I'm sorry, but you probably wont get good at any competitive skill, because that requires competition. You can practice a set of skills, but the most important part of practice is to have a test to see if its paid off. It will surprise you how many people in (probably) your area love playing video games and think there's no one around to play with. Find out more about your local scene, and look for interested gamers to practice with. When I started playing, we just invited everyone we could find who played Smash, most of them stopped coming around because they weren't really interested in getting better, but we definitely ended up with a few friends who really enjoy working hard and practising with us.
- Figure out your next step - Always have something that you are working on. Read up on techs with your character, and try to do it in as many situations as you can, until it really feels good and starts to happen effortlessly. Once you're happy with it, pick something else. Always have something that you are working on. If you do not keep yourself accountable in this regard, you will fall off from practice, because you will just be playing the game over and over and not actually practising anything. Practice is having a specific task and making it easier. Always have something that you are working on. The more skills you can fill your little black book with, the more you will know about what you should be working on next, and the more knowledge you will have to pass on to whoever you are practising with or training.