An advanced technique is a move or combo that can be pulled off by all characters or specific characters and is not recognized as an official move (in Nintendo guides). These moves are often viewed as glitches but they help your game significantly. It is completely worth your time if you want to get better in general. The terms are various Smash terms that you better get used to. Things such as SHFFL (Short Hop + Fast Fall + L Cancel) and Dair (Down Air). As for the tournament related things, it covers which stages are banned and which ones are acceptable. It is also frequently updated so all the information in it is up to date.
DI, CC, and Teching Guide - DI (Directional Influence), Teching, and Crouch Cancelling are all deeper than usual advanced techniques and can be used for recovery and staying on the field longer than usual. These advanced techniques take control over the frames in Melee. By moving a certain way, you can alter the frames to your advantage. You won't have full control over them but you will have enough control to at least better your game to a degree. Worth a look.
Official Glitch and Secrets Topic - Not much to look forward to in the first post but throughout the thread you will see an abudance of various glitches you can perform throughout Melee. Such glitches include the famous "Black Hole" glitch and the "Time Freeze"
Melee Master Guide - New to the Game? Check here! - I made this thing myself. Basically, it will give you all you need to know on how to unlock everything as well as a couple of other things. The entire Special Bonus Points List and Trophies List is included. Worth a look whether you be a beginner or a pro.
Hitbox Frames - Hitboxes are specific areas of a character on a more technical level. You need an AR so you can access the Debug Menu and check out the hitboxes for yourself. They deal with the range of a character and how hits land. Check this thread for more information.
Statistics Listings - Very extensive information on the technical aspect of Melee. If you want to go even deeper than the advanced techniques then this is the place you want to look. It talks about the frames in the game and a lot, and I mean A LOT, of other information on the technical stuff in Melee.
PAL Tier List - There are different versions of Melee (1.0, 1.1, 1.2, and PAL) which basically means that frames are a tad different (nothing big like more unlockables or whatever). So to make sure you are on top of the tournament scene, we also have a PAL version of the Tier List. Check it out.
Select a character that best suits your playing style. Do you prefer a slower, but much stronger character such as Ganondorf? Maybe you prefer a fast-moving, agile character, like Sheik or Fox? Are you a lightweight and hard-to-control but deadly character like Jigglypuff, or a slow-moving, long-ranged tank such as Samus? Or maybe you'd prefer a jack-of-all-trades, such as Link, Doc, or Luigi. Play with everyone, and play through the single player modes with every character to find out who you play best with.
A basic and often not understood element of Smash Bros. is how KOs happen. Every stage, as you'll see, has no outer edges. You'll also notice that your damage meter, a percentage, goes up and not down during fights. Both of these features are quite contrary to most fighting games. This is because the main modes of Smash Bros. require you to knock your opponent off the stage. As their percentage rises it could be considered as losing health, but what it really stands for is their ability to be knocked a certain distance. The higher the percentage, the more likely they can be smashed so far from the stage they are KOed or simply can't make it back to the stage.