The global menu, which I hope you see directly above, has links to each section of this website. That menu, plus "site navigation" at the right are the main navigation aids at this site.
The "site map", which is a fairly detailed index of the whole site, provides another way to find content. I often use the site map to jump directly to a page instead of drilling down through the menus. There's also a special security section map.
The "site navigation" block appears on most pages. Blocks change on different pages to match the page context. For example, this page shows "help" and "concise site index" blocks.
You can also search, using "find things at this site," to find what you're looking for. A direct link there always appears in the "site navigation" block.
The "Open link in new window" trick will make site navigation much easier if you want to explore more than one thing at a time. "Tabs" in Firefox, Opera and now IE7 make this kind of navigation even more effective.
"Tabbed browsing" means access to multiple windows within one browser window. Tabs help you keep track of where you're at and where you've been as well as switching quickly between them. All of these browsers are much more secure than IE5 or IE6.