SharePoint Server 2007 is a big thing to get your arms around let alone your brain to fully understand it. Thankfully Microsoft understands this and is continuing to improve its documentation and guidance for implementing a variety of solutions with it.
The TechNet section for Office SharePoint Server 2007 is continually being updated with richer and richer content. Currently my favorite section is the Design and build sites for Office SharePoint Server 2007. With out understanding the concepts here and following the recommended approach to your desired solution mistakes are bond to happen. Even small mistakes in design early on can have costly consequences in the future