It's really COMPLEX, it's fascinating the way it works.
The way to get online with a xbox 360 is to DELETE the GTA 5 Update from today, re-install update and then go to GTA online and WAIT, you may way for hours, I waited for 2 hours to get 8 people, just let the console run.
Then you race Lamar and after that you're free to do whatever the f*ck you want.
Online is amazing. There's TDM, Last man standing (only 1 life), you can bet on your team, there's races, so far, I haven't done any heists.
It's bad ass. Remember, you can get online with Xbox by following those steps, as long as you don't get lagged out of the Tutorial with the racing and 8 players, you're in good shape. Just wait it out from that point until you get 8.