I don't have much experience with Joomla, but I would imagine that you log in, are presented with some kind of dashboard and can then choose the page to edit.
Is there something in particular you are having trouble with?
What have you tried and what isn't working?
There are also loads of tutorials explaining how to do this.
Maybe trying Google might help: http://www.google.com/search?q=edit+a+website+with+joomla