First of all, are you sure it needs to be replaced? Grinding, sticky spots, or looseness would indicate that it is failing, but if it spins smooth and firm, then it is probably fine. If you do replace it, there are a few fancy brands of square taper bottom brackets that are very nice, like Phil Wood, but for commuting, most people recommend a good sealed cartridge bearing bottom bracket, like the Shimano UN55. They are cheap, simple and durable, and easy to service.

As you search, here are the important things to know:
- 68mm shell
- ISO/English threading (the default for most modern bikes)
- JIS Taper
- 110.5mm spindle (or something very close - 110mm is a common size that would work)

For tools, you'd need a bottom bracket removal tool, and a crank puller for square taper cranks. See this article on the basics of the replacement. http://www.parktool.com/blog/repair-...et-service-bbt

-Sam