I've recently started getting back into Smash and desire to play it competitively. First things first, I need to get better at the game. I can already do jump cancel, shield dashing and dash dancing. What other techniques are super important for me to learn? I also want to learn Marth and have him as my main. Any advice on playing him? What professional players who main Marth should I watch? I already enjoy watching professional matches as they are super fun to watch.
To be honest all techniques are pretty important to learn; however some can be more beneficial to a certain character.
B-Reversing
OOS (Out of Shield) Options
Glide Tossing
Perfect Pivots
Auto Cancelling
Edge Trumping...
There's a few that come from the top of my head at the moment. As for Marth advice I don't know Marth at all so I can't give you any tips on him; however we have Character specific boards here so you should try asking that question in the Marth section if you're wanting proper advice on him. Also please be aware that getting better at the game isn't just about learning all the techniques or just watching pro players duke it out. You need experience, patience, and a lot of practice in general if you want to improve overall.
On top of that you really want to study your character as in-depth as you can.
You need to know their pros and cons, Match ups, and anything that is specifically beneficial to them. Skill isn't going to mean much if you don't know how your character works or what to do against certain characters or situations. Knowledge helps, too.
As for getting into the Competitive scene a good start would be to make attempts to find any locals near you. Don't get disheartened however if you can't locate any close by. If you cannot you can always try looking on Discord. There's surprisingly a lot of Smash Discords out there ranging from Character specific ones to State specific groups. In fact, here's this nifty little site here:
SMASHCORDS
Hopefully this helps in someway or form. Good luck.