Some very good answers here, the following would be mine, which includes some things previously suggested; also I have more than one thing:
1) Asked questions when I had them-- if you are afraid it's a stupid question now, it'll be a lot stupider three years from now. Don't be afraid to admit not knowing something. If you do it EARLY, you won't feel silly later having to ask.
2) Learned theory as I went; learning piano is one of the best ways to do this and has the advantage of giving you another instrument you are capable of playing, even if not to performance levels. VERY helpful.
3) Played along to pre-recorded music, to learn the parts, and also get a feel for consistent time. Learning songs and passages you LIKE keeps you interested and motivated.
3) Internalized my sense of time. Biggest one after learning theory, IMO. Started to think of "time" as a framework to hang notes on, rather than thinking of music as a sequence of notes. This way, no matter if you go off the rails, you are right back on it at the start of a bar. That clock should always be going in your HEAD, it's not in your hands. Learned not to rush or drag, with or without a drummer. Playing with a metronome or drum machine, but also spacing out the click help-- as in, go down to 2 clicks per measure, then switching where those clicks are (1/3 vs 2/4), then going to one click per measure, and so on. See Victor Wooten on this subject.
4) Learned about "the beat" and playing ahead of, on, or behind it.
5) Play, play play. Gigs, experience. The better the people you play with, the more it allows you to develop and excel.
6) Focused on the quality of the note-- there are literally hundreds of ways to play a single note in terms of touch, time, what part of your finger you're using, etc.
That's my half-dozen item "most important thing" list.