Unless you already know the benefits/drawbacks of partitioning, then I think the most user-friendly way to go is one, single partition.
Partitioning is, in a nutshell, a way to make one, physical hard drive appear to the user as multiple, physical hard drives. It helps to separate data for multiple purposes, and there are "power-user" type things you can do to a single partition when you have multiple partitions that you would have to do to the entire disk if you had a single partition setup.
It can be useful and "neat," but for the casual computer user, I don't recommend it. I recommend a single partition setup, and, if you're on Leopard, using Time Machine.
You can re-partition your drive into one, single partition (if, in fact, you find he partitioned it into multiple partitions) by booting from the Mac OS X Install/Restore CD/DVD and following these directions:
http://docs.info.apple.com/article.html?path=Mac/10.4/en/mh14.html
Those directions are for 10.4, but the important stuff that you do when you launch Disk Utility is very similar with Mac OS X 10.5.