The chord Bm/A# that is shown here contains the notes: A#, D, F#, B. The root of the chord is B (The lowest sound - the bass - is A#). The scale from which the chord is derived is Minor triad (1–♭3–5), and the degrees of the scale are: 1, ♭3, 5.
To play the chord Bm/A# shown above, place your fingers as follows:
On string number 4 (the thickest), on fret number 3 with finger number 2 (middle finger)
On string number 3, on fret number 2 with finger number 1 (index finger)
On string number 2, on fret number 2 with finger number 1 (index finger)
On string number 1 (the thinnest), on fret number 2 with finger number 1 (index finger)
Finger numer 1 presses multiple strings as a barre.