When you order Find you'll get the URL for your index. Openings should be just standard rules for HTTP (or HTTPS if you choose to communicate with Find over SSL).
As Johan mentions, EPiServer by default uses HTTP (80) or HTTPS (443) depending on your setup. Also if every port is closed, you will need to open for SQL Server (1433) and optionally for SMTP (25) if you're going to use outgoing mail from your web server.
There is a nice tool called zenmap (nmap) that you can use to scan ports btw. I use it to check that port openings have actually been done by hosting company :)
We have a new web server (not operated by us), by default every port is closed. We need to make an order to the hosting exactly which port and url we need access to. So my question is which ports and url:s do we need to open in the firewall to get the epi site working 100% with find as search engine?