To elaborate a bit more: You need to associate a floating IP with the instance, and you need the name of a user account on the instance. You also need to add port 22 to one of the instance's security groups (often, there is only one security group named "default"). Then run ssh <floating ip> <username>.

$ nova floating-ip-create
$ nova floating-ip-associate <instance name or UUID> <IP address>  
$ nova secgroup-add-rule default tcp 22 22 0.0.0.0/0
$ ssh <IP address> -l <user>

Open the port 22 in the security group that is attached to your instance. Install puTTy (http://www.putty.org/) and connect the server using the floating IP address, username and password