To get your Minecraft server running online, you will first need to enable port forwarding, to forward the traffic from the internet to the game. Minecraft uses port 25565. Once this port is open, you may also want to use an static IP address, since you will most likely turn off your computer often and the default configuration of your router is to assign a new IP address in every start-up, and you will not want to change the server's IP address each time you want to play the game.
Last but not least, make sure to double-check everything is in place, and then give your friends your external IP address so they can use it to log in to your server!