Joomla is actually pretty easy once you understand its structure. To change content you must login as the administrator, usually at:
http://www.yourdomain.com/administrator/Joomla has Sections and Categories. You first create Sections. Then you create Categories and add each to a Section. Think of it like the forum order here.
Once you have sections and Categories you can go to "Add New Content". There you write the content for all your pages. You add each Content to a Category in a Section.
In the "Add New Content" area you find a little box on the right side that says "Show on Front Page:". Check this to let your content appear on your front page.
You can also add content later to the frontpage by going to "Content Items Manager". There you can simply put a green hook on the content that should appear on your front page.
I hope this helps.
Guido