First, confirm that you have a square taper crankset. A lot of low end bikes used to come with one piece cranks that have an entirely different bottom bracket shell than a square taper BB would use.

Second, square taper cranks require a specific BB length in order to get the correct chainline. The crankset you are considering is a track crank which means that recommend BB length will be too narrow for your bike (assuming it's not spaced at 120mm at the rear dropouts). Determining what length spindle you need will be a matter of subtracting a typical track chainline from your desired chainline measurement and then adding twice that dimension to the BB length spec'd for the track crank (assuming a symetrical axle). Some BB axles are asymetrical so you need to be careful otherwise you'll end up with an imperfect chainline. Consult www.sheldonbrown.com for more info.