Conveniently, guitars are set up so major scales can be played using all four fingers on four consecutive frets, to play two octaves. Minor scales can be played similarly, with only one slip down a fret on the 3rd string. All this assumes you start on the bottom string, and work up to the same fret on the top.
The obvious (ubiquitous?) scales that work well on guitar are the pentatonics - maj. and min. Again, they can be played bottom to top and it soon becomes apparent that the notes of, say, C maj. are virtually those of A min.
All so far work wherever you start on the fretboard. Start on 5th fret and it'll be A, 7th, will be B, etc. Put in a couple of blues notes on the way with the pents, and you've got the two blues scales.
So far, no modes! One way to look at these is to think, for instance, that Dorian starts on the 2nd note of a major. So, you could play a major scale, omitting the 1st note, and putting an extra one 2 frets higher on the top. Similarly, for Locrian, start a fret low, and stop a fret low at the top.So, using C maj, but starting on 10th fret, there's D Dorian, or starting on 7th fret, there's B Locrian.
There are loads more ways /places to play scales, but it's a start! Since the patterns are the same, it only matters where you play when you need to be in a particular key.
As far as what scale notes for which song - blues or min.pent for blues, maj. pent. for country, full maj. for most pop songs are good starting places.
As far as knowing each note name on each string on each fret is concerned, I feel that the relative positions and sounds are more important. Others will disagree, but in one's playing, I think far more people play knowing that the next note is 'just there' rather than 'I'm in C#min., I've just played x, so I need to find y to play next'.