Confused about SSL certificates needed for windows proxy

I am completely new to security and I'm confused about what kind and how many SSL certificates do I need to buy.

I have a REST API server running on a windows server 2012. It's currently listening to HTTP port 80. Since I'm going to implement token authentication for the mobile clients, I want to install the Web Application Proxy on my server, so that I can close port 80 and redirect HTTPS requests to my listener at port 80.

When I try to install the Web Application Proxy, it says it requires 1 - A valid Federated Services server running on a different machine, 2 - An SSL certificate.

So I took a virtual machine and installed Active Directory and I was trying to install the FS service, when I found out that FS also asks another (?) SSL certificate.

So, Do I need two SSL certificates, or a multi-subdomain SSL certificate? Are them the same type of certificate?

Thanks for any help.