Ok. You need to know something first, although it's not exactly the answer you might want to hear. Music theory can't tell you the right notes or right scale to use to create music. I can answer your question to some degree and say that starting on one of the notes of the chord your playing is always a safe bet. Example. If the solo starts on an A minor chord; A, C, or E is a good first note for the solo. The majority of the time this is the case, but if you look at all of the music written; all 12 notes have been used over any given chord. So technically you can start on any note, it all depends on what sounds good to you. Music theory will definitely make you a better musician, but it won't give you the blueprints to make great songs.