Circle of fifths
Pick a major key: discover how many accidentals it has, the relative minor and the chords that make it up.
What it's for
The circle of fifths is the key tool of music theory: it shows keys, their accidentals (sharps/flats) and the «family» chords that sound good together.
How to use it
Once you pick a key, you see the key signature, the relative minor (sharing the same notes) and the I-ii-iii-IV-V-vi degrees, the most-used chords in that key.