Fortunately, there is no shortage of info & instructions to do this. Here are a couple of links that explain how to use Exchange diagnostic logging:
For Exchange performance info, one of my favorite tools is Spotlight on Exchange by Quest Software. It is very graphical and “pretty” and does a good job. However, it isn’t going to give you the raw communications detail like the diagnostic logging will.
OH – I just realized that you are using Exchange 5.5, not 2003. Here are some links to configuring diagnostic logging on 5.5:
All the best to you!