You do not need an average score to join a bowling league. There are different types of leagues that you can join. The bowling alley has competitive leagues that high score bowlers bowl in and they are called scratch league. A scratch league is for bowlers that have no handicap or they average is 210 to 220 or higher. Most people just join an handicap league. Handicap are pins that you get with your average. So lets say you have an average of 150. Your handicap could be 70 pins. So if you can’t hit that 150 every game, you have the handicap for help. High average bowlers could be 5 to 10 pins so they have bowl and high game.
So you do not need a score to join a bowling league, just make sure that you join an handicap league where anyone at any level can join. Over time you will get better at bowling and you handicap will come down but the worst you bowl, the more handicap you will get.