Er, theory is kinda useful, lol. Sure, training your ear is the most important thing, but knowing the musical theory is a good shortcut to doing that. You can start breaking the rules once you actually know them ;p, and i don't know anyone who's learned a bit of musical theory and then said it was a waste of their time.
It's not actually a whole lot of effort either. Especially on guitar, when most stuff can be learned in chord and scale shapes. On most instruments you don't get that luxury.
If you still want to take a casual approach and just learn songs, then learn stuff which you'll pick up some idea of how scales or chords work from passively. I'd suggest actually learning the whole of stairway to heaven if you've already started, since there's some pretty cool things to be learned in there, and learning the solo will give you a really firm grounding in pentatonic scales in a few positions.
In fact, finish learning all the songs you've started before taking on new stuff. If someone wants you to do a song and you can only play the first 30 seconds you're gonna look pretty crap lol.
But seriously.. a little bit of theory and you'll be able to make up stuff on the spot for hours that will sound just fine to casual listeners. Not saying you should go out and learn what an e7sharp9 chord is in 3 positions and how to use it, but you should be able to string a few open chords and a couple of melodies together on the spot by now, really.