A bottom bracket has to fit both your chainset and your frame. There are currently four chainset/bottom bracket interfaces that you're likely to come across. If your chainset is made by Shimano and it has splines, then you need an Octalink bottom bracket. If your chainset is NOT made by Shimano and it has splines, it's almost certainly ISIS. There are a very few non-Shimano chainsets with Shimano Octalink interfaces so, to be sure, count the splines - ISIS has 10, Octalink has (surprise, surprise) 8. No matter which brand of chainset you have, if it has a square interface, it will require a traditional square taper bottom bracket. If you have one of the new Hollowtech II systems, you will need to choose the exact bottom bracket that matches your chainsets.

The other information you'll need is the shell width and axle length. To find the shell width, measure the distance accross the outside of the bottom bracket tube on the frame. Correct axle length is crucial to your chain line and the function of your front derailleur. It is very hard to determine without reference to the bottom bracket supplied with the frame. In short, the easiest way to work out what shell width and axle length you require is to choose the same as was originally fitted to the frame although, if you're changing chainset at the same time, this may not be perfect! Stick to the same interface type and brand and you should be OK.