If you have a threaded square taper bottom bracket you need to keep it in order to use your current crankset. Cranksets are usually not something that is worth the upgrade if you are just looking for a way to spend some extra cash. In order to measure your spindle, you need to remove the crank arms with an extractor. If you've done that and measured your spindle at 109.5, it is probably actually a 110mm and you want to keep that size in order to maintain your current chain line. The width of your BB shell will be either 68, 70, or 73. 68 is pretty common. So to keep everything the same and just replace your bottom bracket, you need a BB for a 68mm shell with a 110mm spindle. In order to switch to an FSA crank, it depend on if it is a square taper crank or a crank with a spindle, like 24mm for example. In that case, you need an external bottom bracket that is for a 68mm bb shell but accepts a 24mm (or whatever size) crank spindle. You should also make sure you know what crank arm length and gearing you need or want. You wil also need a splined BB tool to remove your current one and replace it with an identical one, and you will need external bb tool if you switch.

If you are on a budget, it's probably best to keep things cheap and simple by just replacing it with an identical one. Make sure you watch a video or two, it can be easy to strip BB splines.