Help Clearing Up Some Playing In Key/theory ?'s

Ok... I am having a tough time understanding where to start with theory. I played for about 10 years. Even though I dont know any theory I have a good ear and I can still write some decent stuff. But I can really see where its holding me back not knowing this stuff. But thats why I joined this site... To fix those problems. So if you could help me I have a few questions.

Say I start with this riff... this is something I wrote for my band, its a metalcore type riff along the lines of as I lay dying:

tuning: cgcfad



C -------------------------------------------------------]----------------------------------
G ---12---10---8---10---12108---------------------]---8----7----5---7----875---------
C 99---99---99--99---99---------12-10-8-10-12-]55--55--55--55--55-----87578--

*bottom set of nines and fives are palm muted...

Well first of all I couldnt tell you any of those notes without starting with the open string/first fret and counting up to them. Thats bad I know... lol

But say I wanted to write some lead on top of that. Or maybe even harmonize parts of that. Where would I start? What key would that riff be in and how do I tell? And once I know that stuff what scales would I be able to play on top of it... and how do I pick and choose them?

sorry the notes got a little messed up in the 2nd half... they need to look like that first section.

Where Do I Start?

Hello I am new here. I have played guitar for about 10 years and I can get around pretty good. I can write decent riffs but I dont know anything behind them. I pretty much play what I think sounds good without any theory behind it. So without being able to read music or know any theory where do I start? This site seems a little overwhelming and I havent really seen something that jumps out at me. I also have really found any beginning lessons that really teach what I need... or I am not looking in the right place. So do I jump into reading notes, learning theory, or memorizing the notes on the fret board? What are a few video you suggest?