Beginners should learn to play scales on simple chord sequences. Take again Band in a Box and choose at "Juke Songs Now" rock and blues styles. You'll see that blues, major and minor scales are enough. Usually, you don't even have to change the scale during the whole song.
The next step: play patterns you like in other roots. For example: you like a certain pattern in E-blues. Try to play it in Flat A, Flat B, Sharp C, Flat E, Sharp F.
Now, choose at "Juke Songs Now" jazz styles. You'll notice the chord sequences demand a more complex approach. You'll have to change roots during the songs or you'll have to play diatonic or melodic minor modes on certain chords.
The point is to practice on as many chord sequences as possible. You'll become a real improviser when you'll be able to create valuable melodic lines on any given chords sequence. To achieve this, you'll have to reach a point where you are so used to scales and modes that you don't have to think about them too much and they don't interrupt the musical flow of your improvisation.