I Need A Mac Crash Course

So, it has been made apparent to me that my birthday present will be a hand-me-down Macbook that my father is sick of.  This is, however, my first Mac!  As thrilled as I am, I know there's probably tons to learn, and while I'm looking forward to tooling around with it, I need to know:

1.  How to protect it.  It's 13 in, so recommendations for sleeves or cases?  What fits a mac well?  My PC is a beast compared to the MacBook, so it won't really fit in the stuff I own.

2.  What programs do I need to survive?  I'm interested in Scrivener as a writer's app; how does this compare to Word?  What's a good Twitter platform?

3.  I feel like there are a million questions I should be asking and don't know to ask yet; anyone have any other advice for me?

I'm so excited!!!  :-D