I found this neat tool online to save me having to draw out a circle of fifths myself, and clicking on the key you are writing in even shows you the chord 'family'. Since I'm writing in F minor I clicked on that but I'm unfamiliar with the small degree-like symbol that came up next to the ii on Gm. What does it mean? Thanks in advance.
The little circle means "diminished" - so this is a diminished chord(See: https://en.m.wikipedia.org/wiki/Chord_notation)
A diminished chord is a triad with two minor thirds above the root - or you can think of it as a minor chord with a flatted fifth.
You can read more about diminished triads on Wikipedia: https://en.m.wikipedia.org/wiki/Diminished_triad
It is the symbol for a diminished chord. The ii chord in F minor is G-Bb-Db, a triad with a minor third and a diminished 5th. That is the construction for any diminished triad but it is diatonic to the key of F minor. It is also diatonic to its relative major, Ab, in which case it would be a viio.
Adding a little wider information to the fact that 'o' means a diminished chord - built on the 2nd degree of the minor scale (and 7th degree of the major). Because of the way the notes fall, there will be a m3 and a diminished 5 - hence the name.
In any given key, there will be seven different chords produced from the seven different notes. Each chord having its root on one note from the scale, and two thirds 'piled' on top of that. All notes being diatonic - from the key/scale.
So, that will produce three major triads - I, IV and V, and three minors ii, iii and vi. In key C, it's C Dm, Em, F, G, Am, and the one in question B °.