I've found the easiest way to visualize melodic intervals is in a linear fashion along a single string. Let's use the 2nd string (B) and start with our lowest-sounding pitch being the C note at the 1st fret on the B-string.This chart will give you the details:

NOTE	NUMBER OF FRETS FROM C	INTERVAL
C	0	Perfect Unison
C# / Db	1	Minor 2nd
D	2	Major 2nd
D# / Eb	3	Minor 3rd
E	4	Major 3rd
F	5	Perfect 4th
F# / Gb	6	Augmented 4th / Diminished 5th
G	7	Perfect 5th
G# / Ab	8	Minor 6th
A	9	Major 6th
A# / Bb	10	Minor 7th
B	11	Major 7th
C	12	Perfect Octave
C# / Db	13	Minor 9th
D	14	Major 9th
D# / Eb	15	Minor 10th
E	16	Major 10th
F	17	Perfect 11th
F# / Gb	18	Augmented 11th / Diminished 12th
G	19	Perfect 12th
G# / Ab	20	Minor 13th
A	21	Major 13th

(The slash notes are enharmonic equivalent notes—Identical pitch, but different note names)
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