It can be confusing but, all due respect to advice above about going to bike shop, you can figure it out and the best way to do that is to do it yourself, which, in my experience, sometimes means making mistakes. Not sure I understand the point of this forum if novices are supposed to go to bike shops and not ask any questions. You just need to remember, as Fietsbob notes, its the crank that dictates the spindle, not the bottom bracket. If you are going with three piece bottom bracket, not only will the crankset tell you what spindle length to use (crankset manual will have that info) but also the type of spindle interface (i.e. part of the spindle that physicaly interfaces with the crank) of which there a few variations depending make of crank and bb (e.g. square taper, octalink, ISIS splined). Going with a crank with integrated spindle and external bearing BB makes things less complicated since the spindle is built into the crankset so you cant get one that isn't a match.