I would say that responding to a PM is courteous, but not required. I wouldn't worry about your PMs unnecessarily clogging the system. It's just a few electrons.
There isn't a site tutorial, but don't be afraid to play around with the features...you won't hurt anything.
Credits are earned by being active on the site, i.e. logging in, posting, etc. You can use those credits to bet on CyBookie events. Just click on CyBookie on the left Site Navigation menu, or the link at the top of the site.
As always, don't be afraid to ask the community or PM a mod if you have any questions.
