Unless your playing some crazy Jazz or something, you probably won't have to worry about changing scales mid solo, unless ofcourse you song changes keys in the middle of the solo, which it probably wont.
As for knowing what solo to use, it helps to know what key your song is in, and what kind of music your playing. I'f I'm playing a slow song in the key of D minor, I could use a D minor scale for soloing. If I'm playing a rock song in the Key of A, I could use an A blues scale to solo with. Its all pretty basic. Use the scale which most corresponds to the song, especially the key signature and mood.