Could somebody ease explain how to use skype on the iPhone to me please?

I have had the skype app on my iPhone for ages but have only recently wanted to use it. Is it free for me to use over wifi? What are skype credits for? It says I don't have a skype number? Do I need one? How do I get one?
I must sound like a bit of a tard. Lol. Any help will be appreciated.
Thanks.

Anonymous2011-06-22T13:21:00Z

Download the Skype app.
Create an account.
Get friends.
Check to see if any friends are online.
If a friend is online, click on them.
Then press call.

The end. (;