what scales should i learn to use?
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Pentatonic scale, really simple, great for inprovisation. Also learn what modes are.
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I prefer "A" harmonic myself, but to get a good feel for soloing in "Metal" driven songs, it's best to pla the song over and over, and while you play throw in some fills here and there so you can get a feel for what sounds good with the song, when you do write down what you think sounded good.
After you've done so and you feel comfortable with what you play, you can build solos from the fills that you've placed in the song.
ex. I like to (as a warm up) play a 4 chord progression (in dropped-D) after i've played through the proggression twice i begin to throw in fills using the top 3 strings, using the Penetonic scale, go back to the chording then do it again except i change from the top three strings to a simple run from top to... whatever sounds right...

Soloing is just a form of experimentation: try new things, Find the right sound, Get the desired result.

Hope that helps
"...I tell them if its sounds good it's good, no need to change..."

Yngwie Malmsteen-
Correct, the harmonic minor and phrygian runs are most abused and mixing them in between w/ diminished runs for passages. fast arpeggios are also a must and a scalloped fretboard!