You can either use an A shape with your pinky, ring finger and middle finger slide it up one fret and bar the first fret. If you find it hard to hold all the strings with your index finger you can alternatively hold down the first string on the first fret. I dont know if your talking about the chord of Bb or the if the key is in Bb. If the Key is in Bb a cop out way to play the chords, however very usefull, is to capo on the 3rd fret and play in G. When you play G that will become Bb because of the Capo. But if it is just the chord the above ways are best. I've not come across any easi ways to play Bb if I'm honest except a power chord but that type of chord only will suit certain styles. Try the way I've said above, if your not used to baring or the stretch you need you will get it by keep on doing it. Hope that helps. Let me know