Wrote this on reddit the other day to someone asking about techniques to learn, I'll paste it here:
L-canceling and
SHFFLing are absolutely essential and should be the first things you learn. In PM your character will flash when you successfully L-cancel to signal that you did it correctly.
Ness, Lucas, and Yoshi should master
Double Jump Canceling, which is even better than SHFFLing. Peach and Mewtwo get to upgrade even further to
Float Canceling.
DI and
Teching are vital survival tools. They can save your life.
Dashdancing and
Wavedashing are important movement techniques. Dashdancing is very easy to incorporate into your game as a beginner, while Wavedashes can be a bit trickier. Start by using it as a substitute for rolling, and practice doing it out of shield. Once that's second nature you'll get a feel for where else it's good for.
When running, most characters should use
Jump-canceled Grabs instead of slower and riskier dash grabs. Also use
DACUS as an alternative to regular running upsmashes, though note that DK, Diddy, and Kirby cannot do this in PM. For everyone else, PM has changed the timing window for DACUS to exactly two frames and it doesn't matter whether you use A or Z.
If you play Peach, Link, Toon Link, Snake, or ROB, learn
Glide Tossing and Aerial Glide Toss (same thing but with airdodges). AGT basically lets you airdodge and act out of it, making for a very handy recovery tool. Any character can do it, but only when there are items on the field so you'll only get a chance to when fighting against those same characters. So even if you don't main them it may still be handy to learn for those matchups, but consider it a low priority.
For style points, learn to
Taunt Cancel. This is actually important for Samus, since she can switch beams faster with it. For everyone else, it's just cool.
