it really will depend on how your site is structured now.
if it just straight static content that has no products etc - you could quickly recreate these pages in the magento cms.
then you just need to configure the ecommerce part of magento add products, set up shipping, payment processing etc etc etc and off you go.
if your site already has products / ecommerce functionality then you will need to either copy these products into magento with all of the associated product information or do some type of import although I doubt that is feasable if you just have static html pages.
you will also need to 301 redirect your old pages to the new magento powered pages.
you could just add magento to your existing site as the store part of the site
let us know a bit more about how your site is structured and one of us will be able to give a bit more help