You'll need to do two things, firstly you need to port forward if you have a router, there's plenty of guides on the internet for that, just google "port forwarding for *INSERT YOUR ROUTER BRAND AND MODEL HERE*", the port would be 8333 and 8332 TCP.
Secondly, you may need to add to your bitcoin.conf:
This will make sure that any IP can connect, not just ones on your LAN, or localhost.
WARNING: You might want to configure it to the exact IPs, or an IP range of where you'll normally connect from, this will protect you from hackers brute forcing your server