[support] multiple sites on one domain

I have a single user system and need to work on 2 quite different sites on my home pc before transferring them to the isp . Where can I find instructions on how to set up and work on 2 quite separate web pages. I guess they will use the same database so how would I separate each site so the components and database info can be transferred to a live ISP site. At present I have separate drupal installations. I believe there is a way to have one theme-set as the shop front - main page that everyone sees and another theme-set of pages for members. Where can I get instructions for beginners about this please. Off subject-- Is it worth going over to Drupal 7 now instead of having to upgrade down the track. TIA Roger