Make sure you have a server set up first!
The video may help you.
1. open into system preferences
2. go in to network and click advanced
3. click TCP/IP
4. change configure IPv4 to using DHCP with manual address
5. type 192.168.0.X where X is a value, 9 or 10 should do unless you have more devices in your house
6. go into safari and type 192.168.0.1 in the top of your browser, the username should be admin and the password should be password
7. go to the portforwarding/port triggering screen (don't worry if it doesn't say that exactly)
8. press add custom service call it minecraft or something
9. make sure it is TCP/UDP and both ports are 25565
10. press add and at the top select minecraft or what you called it, allow always and type 192.168.0.X the x is the number you chose
11. press apply and then again
12. open your server folder and open the properties
13. make sure you leave the ip blank if you want to edit anything else and leave the port to 25565
14. start your server, the ip it gives you should be *:25565, this means you have done it right
15. go to http://www.whatsmyip.org and copy the ip, your friends will use that with :25565 on the end to connect
Be aware it may change every time you reconnect to the internet or each day
you are done! your friends can now join, if you still have problems watch the video and read the tutorial again. If you still have problems tell me in the comments