If you have not used MrBayes before, or would like to get a more thorough introduction than you have received previously, we suggest you walk through the MrBayes 3.2 tutorial provided here.
To use the tutorial, Mac users will need to download the files primates.nex and cynmix.nex. Ubuntu users should already have these files in ~/Molevol/mrbayes/examples.
More experienced users might want to explore the information and links provided on the software page for MrBayes.