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Well there are a few tings you need.

-Fast internet. (Be able to support you and your friends connecting to you.
-A decent computer to be able to run both the server and you playing or 2 computers.
-Port forward. (You must forward 25565 port to run the server. Mos people use hamachi for small group of friends but I personally think it suck. cause you will get a lot of lag.)
-Bukkit or Minecraft server (Minecraft server lets you play minecraft with friends. Bukkit lets you use mods like flying and permissions and that sort stuff.)