If you know of additional resources useful to beginning developers, please feel free to edit this document to add them.
If you install Domino Designer, and open the built-in help, there's a pretty good introduction to how development is done.
This wiki contains lots of good information for beginners. You might just read the whole FAQs Category; you will almost certainly run in to all these issues at some point.
Domino Designer 6 A Developer's Handbook, though a little dated, still covers the basics. Ignore the sections on web development, since this is better done using XPages now.
Every developer should read Performance basics for developers (whitepaper) to avoid creating applications which get unacceptably slow as more documents are added.