I'm extremely new to this and could use some help understanding the basics of creating a database for a login page and applying server behaviors to webpages. We plan on pre-assigning usernames & passwords so that only certain individuals are allowed access to private pages designed using Dreamweaver MX. I've gone through the sofware help but don't seem to find a good enough explanation of how to choose which document type to select or which program to use to create the database. Could anyone point me in the right direction to a book or tutorial or actually explain what to do? Any and all help is appreciated.

Generally I would say avoid MS Access for all but personal databases - it has problems dealing with more than a few connections at once. So mysql or SQL server would be good choices - depending on your budget. To see what micro$oft say check this out www.microsoft.com/sql/techinfo/planning/SQLAccess.asp

For the choice of document type, that's down to which language you feel happiest working with. To a great extent you wont need to get your hands dirty with the actual code as DMX will deal with it for you - but if you have to delve into the code it would help if you understand the syntax of the language i.e. JavaScript uses the plus sign '+' (which wont come out in this text!) whereas VBScript uses ampersand ‘&’ to concatenate.
As with any web application it is the requirements of the project that should dictate the tools used.