Like many open-source projects, our official documentation is not the best it could be, but please go there first if you are new to OpenNMS or if you have questions.
There also is a categorized compilation of most documentation available on this wiki here. Please feel free to add and edit links and also their targets according to the constant change of OpenNMS itself. Only a well documented project will be successful with people that are new to it. If you have experience with OpenNMS please contribute here. Also if you are just starting and are installing OpenNMS onto a new platform or version of a certain OS, please write an install how-to with all the tricky stuff you found out. This way others will have a running version faster and they themselves can gain experience quicker.
If you don't find something in the official documentation, please also search this wiki (there is a search box on the right-hand side of the page) and search the mailing list archives and Google. If you think you found what might be bug, also search the bug database. There is a lot of information out there that isn't in the official documentation, and most questions can be answered by searching these resources.
Of course, please add content to existing pages in this wiki or create new pages for items that aren't well documented. We'll also gladly accept additions to our official documentation.