If you learn your circle of fifths it is quite trivial, because nearby scales on the circle (i.e. scales that have tonal centers a fifth apart) only add or remove a single black key. F to C (Bb becomes B) to G (F becomes F#) to D (C becomes C#) etc. is very easy and a lot of music will stick to those. Learning scales on guitar (or indeed on an isomorphic keyboard, for that matter) is only slightly harder, the trick there is knowing your solfège so that you can find your way around the semitone intervals and (for guitar) what to expect when you shift to a nearby string.