While it’s true that the intervallic relationships of a major scale always remain the same, there are many ways to play the same scale on a guitar. This is due to both the tuning of the guitar as well as position of the starting note. (See William levitt’s “Modern method for guitar).”
Regarding modes, there are two approaches to understanding modes: relative and parallel. The relative approach may be understood as simply beginning on a different scale degree of the major scale.
For instance, the Dorian mode begins on the second degree of the major scale, Phrygian on the third degree of the major scale, Lydian fourth, etc. The parallel approach involves the alteration of the intervallic relationships of the Major scale. I often separate the parallel approach into two categories: major and minor modes. The major modes are: Ionian, Lidyian, and mixolydian. The minor modes are Dorian, natural minor, and Locrian. Each of these modes can be derived by slightly altering their respective major and minor scales. For instance, the Dorian mode may be spelled like a natural minor scale with a natural or raised 6th degree, if you prefer. So the relationship of whole and half-steps would be as follows: whwwwhw. Before concerning your urself with modes, I would suggest further study of the major and minor scales, as these are the basis for the choral “church” modes and an understanding of chord-scale theory. William Levitt’s book mentioned above is a good place to start for the self-directed learner. However, finding a suitable teacher whom you can work with directly would considerably speed up the process.
Best of luck.