It's NEVER a bad idea to join a league. Just the routine of bowling on a regular basis is a good thing. And don't worry about being good enough -- that's what handicap systems are for

I carried a 137 average in my first league -- we all have to start somewhere.
Handicap varies significantly from league to league. As a general rule, it should be based somewhere near the highest average in the league. If you have a couple low 210s in the league, then 210 should be the basis. Percentage (80%, 90%, etc) is most commonly (from what I see) 90% anymore. The higher the percentage, the more even the playing field. At 126 average, you would get 75 pins per game on a "90% of 210" league, for example.
While there aren't many guidelines for what a beginner league is, the easy way to find one that isn't loaded with high average bowlers is to look at the league recaps posted at your local center and key in on the high average list. If you can find one with no one (aside from subs) over 200 -- it might suit you well as a "beginner" league.
Hope this helps!