What service and IP address do I need to host a server?

I've finally figured out how to port forward but I still need some help.
What service and IP address do I need to host a server?

If your router is asking you for an IP address to forward, and there's both for "External IP" and "Internal IP", for external leave it blank (if you can, otherwise use 0.0.0.0) and for interna write down your internal IP.

I don't know what this "service" could it be, but if it has the option for TCP and UDP, use UDP, or both to be safe.