Okay ive tried lots of stuff and read all over. so im gonna ask this in a short paragraph.

ive heard of music theory.. which I do not know.. and i know some scales but not many and I know some chords, basic and that stuff.

I can play a few songs but I really wanna play something like Ac\Dc back in black.

Do I need to learn a scale or something first? or do people just look at the tab and practice it?
there is few times when the word "need" can be applied to guitar.

so no.. you dont need to learn a scale or something to play a song, although learning a few would certainly help out
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If you want to learn a certain song, just look up the tab in the archives. Music Theory is just the study of what sounds good. It explains what songs/chords/riffs/chord progression/solos are in what key, and what sounds good. People use scales to basically write music. You're probably relatively new to guitar, so just play the guitar and have fun with tabs, maybe look up some finger exercies to help you advance. When you think you are ready, start learning music theory. It helps you better understand what you are playing and how the artists wrote the songs. Once you get good, you will understand keys and start writing your own music.
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I have been playing for about 5 years now. I have never learned a scale or an arpeggio on guitar period. I progresed just by finding songs I wanted to play and practicing them like crazy until I got them right. Each new song you learn will undoubtedly challenge you with a new skill. After so long, it wil just become a natural thing. I can improv solos over about anything now just from remembering lines I have taken in 100's of other songs over the years not by wanking the same scale lines over and over. (*cough*......MALMSTEEN.......*cough*) Just keep finding songs you want to learn and keep your ears open for new and challenging things. Don't just keep playing power chords your whole life. And if you don't belive I can play without learning theory, just give this a watch. http://youtube.com/watch?v=fYU_aYfk-EQ Good luck dude!
Learn the major/minor scale in any case. You don't have to be able to play them at extreme speeds, but just know how they sound and what they are. Although you won't find a use for it yet, it's important to know about stuff like scales from an early point, because that'll make learning to use it MUCH easier later on.
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Reading tab is part of studying actually. It helps you to understand the technical bits and pieces of guitar riffs. But every instrument is different. Music theory covers a whole lot of areas. Here is a link for you.