Total newbie….

I am totally new to all of this but am hoping to find a way to learn how to customize a blog.
I started a blog using wordpress but didn’t find a template that I liked. I just want to customize colours and margin widths etc.
So I have downloaded WordPress thinking that was what I needed to do some tweaking. But now I don’t know where to go next or if there is a next.
I have read some of the online tutorials but am stuck at the very beginning where it asks you to change these areas:

define(‘DB_NAME’, ‘putyourdbnamehere’); // The name of the database
define(‘DB_USER’, ‘usernamehere’); // Your MySQL username
define(‘DB_PASSWORD’, ‘yourpasswordhere’); // …and password

How do I know or find out what my database and MySQL usernames are???

And I’m assuming that the wordpress.com is a compatible hosting site.

As I said, I’m totally new to setting up my own but would like to try.
Am I on the right track?
Any help??