Query, Synopsis, Cover letter? EEK!

Okay, I know how to write the query.
I've figured out how to write the synopsis.
But what the heck do I put in a cover letter?

Yes, I know that should be the easiest out of the three, but.........

It's like my poor son, filling out job applications and he has no job history to report
 because he's never had a job!

Well that's me. I have written my first novel and yet have no history.
Nothing ever published, no awards....

So what do you put on that cover letter?
Do you list the blogs you write?
Your three facebook pages you have?
Your twitter account?
The guest blog you wrote for someone else's blog?