Turning website into multi-site?

Hi

I’m running a WordPress 3.7.1 website, with not a lot of content on it yet, I’m pretty new to wordpress.

I want to be able to eventually have an online store where users can list their own products for sale, like ebay but totally buy it now style, not auction, probably using the WPMU Marketpress Ecommerce plugin, which I’ve just noticed requires multi-site (whatever that is!)

I’m not an WPMU member yet, so can’t ask there, but how can I easily turn my existing site into a multisite installation please?

Is there are really simple way, or do I need to delete the whole website and begin again, this time installing wordpress multisite?

Thanks