The blues is a good entry path for beginner guitarists. If you learn the standard fifteen chords and the blues scale, you’ll be well on your way. However, there’s one crucial piece of additional music vocabulary you need to fully inhabit blues tonality, and that is the mysterious diminished seventh chord.
To make a diminished seventh chord, you start on any note, go up a minor third, then another, then another. Here are the notes in Cdim7:
You can also derive this chord from C7: take each note in the chord except for C and slide it down a half step.
This video explains how dim7 chords relate to other seventh chords. Here are some good guitar fingerings for dim7 chords.