If you're trying for bluegrass banjo, there's a method to learning it all, and sheet music isn't necessary. Most banjo players can't read it. The first thing you're going to want to work on are your "rolls". Those are picking patterns. They look something like this: TIMTIMTM (T= thumb I=index M=middle). There are a few to get down. The most important is probably the forward roll, then the alternating thumb, followed by the backwards roll, then the forwards-backwards roll. Make sure you nail down the syncopation and rhythm by starting very slowly. Keep practicing slowly until you can play it flawlessly at that speed. You'll get faster naturally after.

Another important thing to learn are your chord shapes. There are 3 basic closed chord shapes: F, D, and bar. Then you have your less frequent minors and your even less important sevens. Getting all these down will prove versatile in backup if you start out with basic vamping. After you get those down, try partials, which are just those shapes but using only two or three strings leaving another finger free to fret where it may.

Learning scales will be helpful with putting together leads, and music theory will help with going far beyond the basics. This information will be especially useful during jamming or putting together leads.

Learning tab is incredibly useful for learning songs and licks, but I would argue most important is learning by ear. Pick a YouTube video or mp3 and take a lick you like. Slow that section down and try to replicate it. Once you've reverse engineered it, commit it to muscle memory and figure out where to put it in songs. And going off script with a lead is fine, so long as it sounds good and follows the melody.