I just wanted to point you to a series of funny (comic style) db4o blog posts by gamlerhart which make a great way to get you started with db4o (why not having some fun while you're learning?). I think the blog posts also deal with key issues that you must have in mind if you're new to db4o (activation, object identity, etc).

So, learn and have fun!

1. db4o, The Basics
2. db4o: Activation- & Update-Depth
3. db4o: Object-Identity and High-Level-Caching
4. db4o: Transactions
5. db4o: Persistent Classes Tips
6. db4o: Single Object-Container Concurrency
7. db4o: Queries in Java or Queries Without LINQ
8. db4o: Client-Server and Concurrency

(Thanks a lot gamlerhart!! =)