B flat (Bb) major is one of the barre chords. If you already know B major chord, you should be familiar with the finger position for Bb chord:

As you can probably tell, the only difference is that Bb major shape starts on the 1st fret.
About Bb major chord
- Chord formula: 1 – b3 – 5
- Chord notes: Bb – D – F
- Alternative names:
- BbM, BbMa, Bbmaj, BbΔ
- Common functions:
- IV-chord in F major key
- V-chord in Eb major key
Bb major chord without a barre
Just as the B major chord Bb chord can be played without a barre:

Be careful not to play 5th and 6th strings.