Sharepoint how to give a permission to user to only one site/folder

Hi,

I have a question about Sharepoint 2010.. I guess. Let's assume that I have a Sharepoint where I have 4 sites. Site A, B, C and D.
I want a user "Jack" who works in my company to access only Site B. He has to be able to do changes in the Site B. He must not have permission to view or edit other 3 sites.

Can I have step by step guide?

Thank you :)