SSL Wildcat and domain forwarding
if i buy an SSL Wildcat, will digitalocean know?
and how i can i forward one of my subdomain "dev.7mcity.com" to "www.7mcity.com"
DigitalOcean provides unmanaged hosting. You get full root access to your server. With that power comes the responsibility of configuring it yourself. So DigitalOcean doesn't know that you have a wildcard SSL cert, but you are welcome to use one if you wish, but you'll need to configure your webserver to uses it. Checkout this tutorial for an example. It should point you in the right direction.
Let us know if you run into a more specific problem.