mod_rewrite help

[miriamango]

I know there's a way to do this, but I have absolutely no idea how.

How can I change

username.domainname.com & www.domainname.com/~username

into www.domainname.com/username ?

I think you have to have an .htaccess file but I have no idea where to find that, if I even have one. Any help will be greatly appreciated, thanks in advance!