Hi,
you need to do two things:
[*]Find out your external IP address (careful here, on normal DSL lines it changes every day)
[*]Configure your router to forward traffic to port 27000 directly to your home PCs port 27000
The first is easy: go to a website like https://www.whatismyip.com and see it ;)
The second depends on your router and your internet line. There are countless tutorials in the interwebs for "port forwarding" for every router in existence, so just search for "port forwarding <your router name here>" and you should find something.
It also depends on the internet line because there are lines that use a shared IP address for many users, so if it doesn't work despite the port forwarding it might be this problem and you can call your provider and ask them if you have a shared IP or a public accessible one.
If your friends can connect and you want to get rid of the new IP problem every day look for dynamic DNS providers. There are many and also many free ones and then you get a domain you can give your friends and the PC or the router do the update automagically.
Other options for having a game server for friends is using a provider for this, they are not that expensive and take away all the pesky setup, and using a public server, which are spawning up like zombies in minecraft at night ;)