How much music theroy should I learn, currently starting to learn I,IV,V,VIII major and II,III,VI,VII minor. I guess I should learn some basic scales for fill.
Major, Minor & Pentatonic (the "Blues" Scale) and you should be good to go. Add in the Harmonic Minor if you want to get into that Blackmore 'Snake Charmer' vibe.
I know about 25 songs by tab.
To be brutally honest, if you can't play them without using tab, you don't know them
at all. Put the tab away & play along with them, that way you
will learn them, and they'll stick.
I did my first gig a couple weeks ago and I found I would get lost in the song and its not fun .
If you were trying to get through the gig by relying on tab, I'm not in the least surprised. Most tabs are 'get out of jail cards' at best & are of little or no use in a live situation. It's tough enough trying it with lead sheets & chord charts, with tab I'd lose it altogether.
For a tune to be giggable,
don't practice it until you get it right, practice it until you stop getting it wrong. You'll still get the occasional brainfart when you lunge for the chorus instead of going to the bridge - we all do that - the trick is being able to get back on track ASAP, & you won't do that unless you've really internalised the tune & its arrangement. This means knowing what key it's in and the names of the notes on the fretboard - if you're lost without that map, you're really lost.
Jamming with a drummer is a really good idea, as that's your first target - to lock in with the sticksman. Playing along to recordings helps to cushion the shock too.
As Bredian suggested, fill up your iPod with a whole host of tunes (stuff it full of Sabbath & AC/DC (downtune a semitone for both)), set it to shuffle & crack on.
Keep off the tab sites unless you're absolutely stuck on a section or specific fill - you don't need 'em, really, you don't; you don't need a crutch, you're not lame.
Pete.