Just play whatever sounds good. After you get a few cool things that go together, look into what scale they fit into and use that scale to help you write more along the lines of what you've already got.
In the Oven:

18 Watt!
minor pentatonic never fails XP
just dont abuse it like zakk wylde
and actually learn another scale after you learn it
alot of my friends who play guitar learned it(myself included)
and over use it
Try a bunch of stuff and see which one you like the most, or just acutally play what comes naturally.
it depends what type of song you are writing, obviously
Quote by RPGoof
I feel dirty just reading that word. Grindcore... EW... I need a shower

Quote by stratkat
music is my meth equivalent
Best thing to do is to figure out what you want the solo to sound like. Sing it in your head, make it melodic. Then find the spot on the guitar that makes it easiest to reach all the notes that were in your head.

That's the "position" you will center your solo in; the right scale will show itself to you through practice and patience. Once you "find" your voice in a given song, it all starts to make sense from there. When you've written a cool solo, you will likely find that you have taught yourself some new licks, too!
Have you not ever sat down and tried out these scales and even just put notes together through trial and error and decided what you like?

I don't know why some people think they need to know so much theory in case they're doing it wrong, it's music......use your creativity.
It depends what sounds you like. A blues man will probably use minor pentatonic. A classical guy probably major scales. A spanish gypsy would use spanish gypsy scales. Depends on what floats ya boat.