You pick a bottom bracket to match your crankset and frame:

• The spindle length, type, and taper need to match your crankset.
• The bearings/cups/cartridge need to match your bike's bottom bracket shell dimensions, and threading if applicable.

What kind of crankset do you have?
What kind of bike are you installing it on?

If you don't already have a crankset, pick that first. Then get the right type of bottom bracket to match.