#1
What would it be?

What thing do you know now that you wished you knew when you were a relative beginner? Whether it's something to learn, a particular technique, or just some general nugget of wisdom?

As a relative newcomer I thought this would be a fun topic
#2
Trust my ears...

I've got nothing against tabs. I'm sure my playing advanced faster because of tabs. However, my ear doesn't hear the things it should, because it's too easy to fall back to tabs for "how to play" stuff. It's more than a crutch for me now.

I'm working on developing my ear now..... 20 some years later.
#3
Play harmonica. It weighs a lot less than my 89lb amp...

OK seriously, I would advise my younger self to not worry about playing fast, concentrate on smooth and melodic.
Hmmm...I wonder what this button does...
#4
I would tell myself to practice ear training earlier and become more involved and practice more efficiently earlier one.
Skip the username, call me Billy
#5
Buy gear used and cheap, learn the basics, form a band, and gig as often as possible... everywhere. The more you gig and interact with others musically, the better you will get and you will learn to work an audience.
"Your sound is in your hands as much as anything. It's the way you pick, and the way you hold the guitar, more than it is the amp or the guitar you use." -- Stevie Ray Vaughan

"Anybody can play. The note is only 20 percent. The attitude of the motherfucker who plays it is 80 percent." -- Miles Davis
#6
1)learn how to incorporate dampening and muting from the start - resting your right hand and fingers on strings that aren't played etc. It's what makes great lead playing.

2) get your guitar professionally setup every 6 months.

3) you can't be great at every style - focus your efforts to your taste.

4) learn a set to perform at a gig - have a set of songs that are performance ready.
#7
Practice more!!!

Honestly playing came pretty naturally at first, and so I didn't practice like I should have. Now I don't have the time and I regret it. I'm comfortable with my playing, but I know if I had practiced more I'd be able to melt faces, pull heart strings, and drop undies with the best of them.
I'm just a kickin' and a gougin' in the mud and the blood and the beer.