There are three options for you.
1. Install basic Joomla and create the information architecture of your current site on joomla. After creating the menus and other structure cut and paste all your pages into the content pages.
2. Install basic Joomla, Choose one good free template or buy a commercial template. Create the information structure and put your html content into the pages.
3. Try to customise Joomla template to look like the current website of yours. or catch hold of a custom template developing freelancer and get your custom joomla template done for you. And then finally put the content into the site after creating the information architecture on the new template.
Ram Kumar
www.rheakt.com