I hope you are not new in managing website. If you are new then I will suggest that you go for Wordpress installation and use that for sometime till you get feel of it.
If you are already experienced user then first step is download Joomla guide - um-joomla.pdf available on the joomla site.
These use manual provide detailed instructions.
Then may be in first round go ahead and create one test item for each i.e. menu, category, content etc so that you will get feel of Joomla workflow. Then try to tweak with all other options to find what all is there.
Once you have get the idea then write down (yes on paper) what exactly you want to put on the site.
Now uninstall the Joomla which you have fiddle around and do a fresh installation and then go ahead with developing your site.
I hope this will help.