As above, if you have tools then take the BB out and have a look – it should have the shell width plus axle length printed on it. If not then measure end to end on the axle and across the BB shell on the bike frame.
A common size if 68 BB and 113 axle but yours may not be that.
Once you have the measurements then any square taper BB will fit. Shimano UN52 or UN54 is the default choice for a lot of people as they seem to last for ever.
If you need tools have a look on the on-one site – they seem to have some bargins at the moment.
You’ll need two tools beyond simple allen keys – a crank extractor and a BB tool – the web descriptions shoudl say what type of cranks/BB’s each one tool for.
[edit] have a read of this: http://www.parktool.com/blog/repair-help/cartridge-bearing-type-bottom-bracket-service-bbt
http://www.parktool.com/blog/repair-help/crank-installation-and-removal-square-spindle-type