I think to begin with you need to understand the distance (either half-steps or frets , they are the same) between two notes that form an interval. It is fairly easy and at first you really only need to know them, not necessarily know why they are named the way they are. As you get more into it it should become clear.
Unison = No half-steps (C--> C)
Minor 2nd = 1 half-step/1fret (C--> C#/Db)
Major 2nd = 2 half-steps/2 frets C--> D)
Minor 3rd = 3 half-steps/3 frets (C--> D#/Eb)
Major 3rd = 4 half-steps/4/frets (C--> E)
Perfect 4th = 5 half steps/5frets (C--> F)
Augmented 4th or Diminished 5th = 6 half-steps/6 frets (C--> F#/Gb)
Perfect 5th = 7 half steps/7 frets (C--> G)
Minor 6th = 8 half-steps/8 frets (C--> G#/Ab)
Major 6th = 9 half-steps/9 frets (C --> A)
Minor 7th = 10 half steps/10 frets (C--> A#/Bb)
Major 7th = 11 half-steps/11 frets (C--> B)
Octave = 12 half-steps/12 frets C--> C)

Memorize these and then learn them. Good luck.