I assume this is a standard "cup-and-cone" bottom bracket and readjusting it is pretty straight forward. Park Tool's web site has a tutorial for this job here:

http://www.parktool.com/blog/repair-...racket-service

If it is loose you will need to readjust the "adjustable cup" ( on the non-drive side of the bike, that is the side without the chain) and lockring. If the bottom bracket has not been serviced in a long time, an overhaul, cleaning and relubeing should be considered.

In either case some specialty tools make the job a lot easier but there are some work-arounds. .