Open the correct port on your router/modem, default port for Minecraft is 25565 I do believe but check in the server settings file. Opening ports is easy once you have figured out how to access your router. The best way to figure out your router's internal ip so you can access it is to open command prompt and type ipconfig and hit enter. A big list of crap will appear. It will use confuse and you may hurt yourself in your confusion. Once you have regained your sanity look right at the top of all the random crap and you will see something like this:
IPv4 Address.............:*some numbers*
Subnet Mask..............:*some numbers*
Default Gateway..........:*some numbers*
The number next to "Default Gateway" is the internal ip address of your router (its address on your local network) if you type this into your web browser you will be able to access all the settings on your router. This will be different depending on what router you have so I cant really help you here. You will also need to go into your local area connection settings to set a static internal ip. You will need the numbers you got earlier for this. Find the local area connection status window and go into properties. Under the networking tab there is a list of tick-boxes. Untick "Internet Protocol Version 6". Highlight "Internet Protocol Version 4" and click properties. Select "use the following IP address" and enter the relevant info. hit "ok" and go into your router settings as described earlier. In here you will need to make the ip address you put into the local connection settings a static one. How you do this depends on you router. Then you need to open the correct port number to that ip address. Again this depend on your router. Google it. Remember to look in the server properties file for the correct number (you should have downloaded the server files already). Once you have figured it out you need to find out what your external ip address is. You can google this cos there are websites to tell you. Finally run the .exe file for the server and your friends can join using your external ip. You will have to join the server using your internal ip, however.
