Good luck on finding a resource. In my experience (more in the form of books than the internet) resources are either way above your head or really far beneath you. I have rarely come across something that really helped me out. One such thing was The Chord Wheel by Jim Fleser. It cost about $10 and it is just a circle of fifths with all the notes spelled out for you.
Until you find that comprehensive guide that you're looking for, you could perhaps just start dabbling. As far as I'm concerned, the minor scale is the backbone of music. That's why those dots are on your fretboard; they represent the notes of the minor pentatonic scale. And the minor scale isn't hard to do. Here's how I play the minor scale on a re-entrant uke:
C string: fret 2, then 4, then 5
E string: fret 3, then 5, then 6
A string: fret 3, then 5
That, in fact, is D minor because the first note we pluck is D. That's it. Now all you have to do is practice until you can play those notes back-to-back, backwards and forwards, somewhat quickly. Then you start messing around with those notes. Alter the phrasing: play two of the notes, pause significantly, play one more, pause, play three more, pause, play the last two--and you have a song! Or start skipping notes or playing them in a different order. Dude, now you're improvising!
I will only say one more thing and then stop before this gets too long. Here's the thing. That scale is completely movable. Look at what the fingers do: you play the index, ring, and pinky fingers on the C string, then you move up a fret and do the same thing on the E string, then you move to the A string and play the index and ring fingers. That pattern works wherever you go on the fretboard. For example, if you begin the pattern on the 8th fret, instead of the 2nd, then you're playing Ab minor because you start on Ab. So one pattern gets you all the minor scales.
