Every WordPress site is capable of Multisite from the perspective of the files contained in each. What has to happen is to run some scripts that will prepare the database and file structure, as well as deliver the necessary changes that you will need to make in your .htaccess and wp-config.php files. Everything that you'll need is explained in this page of the codex:
Essentially, you'll trigger a change by adding a constant to your wp-config.php:
define( 'WP_ALLOW_MULTISITE', true );
Once you've saved that in the file, you need to go to the Tools menu in the dashboard and run the Network setup script. When that is finished, you will be given two snippets of code to place in the htaccess and wp-config respectively. Once you've done that, you'll need to login again, and you will now have a network admin separate from the dashboard for your main site.