Circle of fifths.
C major
No sharps or flats
Chords in this key
Six of them sit together on the circle, in a coloured wedge. Play one to hear it.
Hear it: the pop progression
Behind countless hits. Hopeful, then bittersweet. In C major.
Next door
Inner ring: the same notes and all seven chords, with a sadder home.
B becomes B♭. Shares C, Dm, F and Am.
F becomes F♯. Shares C, Em, G and Am.
Why it helps
To find a song's key, look at its chords. C, F, G and Am all sit in this wedge, so a song built from them is in C major, or in A minor if it keeps coming home to Am.
Keys side by side change only one note, so they share most of their chords. That is why a song can drift to the key next door without a bump.
On guitar the friendliest keys are C major, G major, A minor and E minor: almost every chord in them is one of the first chords people learn.