You can open your own Server this way (everyone who knows your ip can join):
1.) Download the multiplayer-server .exe (if you are running on windows) from

(put it into a folder and doubleclick it)
2.) Open the port 25565 on your firewall (i cant tell you how you can do that because it differs from firewall to firewall). Just google it.
3.) Tell your friends your IP (

)
Still questions? Add me on Skype (phlinther1) and I can give you further advice.