I made a couple websites on a drupal multisite setup.. and I want to start using https. And.. I'm just lost. Can anyone help me get started?

So, here's the details of how I have things set up..

So I have like a drupal 7 install on ubuntu, with 3 sites.. all in the var/www/html/sites folder, so for example, the folders are named

site1.example.com
site2.example.com
site3.example.com

and the sites.php file directs the requests so like

$sites = array(
    'domain1.com' => 'site1.example.com',
    'domain2.com' => 'site2.example.com',
    etc
);

and as far as dns is concerned, each domain just points at the IP of the ubuntu server and drupal handles the rest.

All that is so far so good. Got several working sites.. each domain goes to the right site.. etc. But they use http. How.. do I go about.. getting a certificate that handles the 3 sites so that they can use https? I've not set this up before.. I've been trying to read through whatever I can find.. but I'm confused! Sorry, not sure what to do, I'm just .. lost!