I believe your question is how to capture the correct key and guitar tones of a particular song. First determine to key (usually by the first chord of the song - if first chord is C major, then the song is most likely in C major and you'd use the C major scale. If first chord is E minor, then it's most likely in key of E minor.)

Next, try to match the guitar tone by determining if it is clean (no distortion) or overdriven (with distortion) and what level of distortion (a little like AC/DC or a bit more like Slayer). Also listen for any effects the guitar might be using (like reverb, delay, chorus, etc) and adjust accordingly.

A good modeling amp would have a library of tonal options to choose from. There's a bunch currently on the market (Yamaha 'THR10' and Positive Grid's 'Spark' amp are two modeling amps to check out).

I hope this helps :-)

Keep those fingers flying!!

-Dave