Go to http://canihostaminecraftserver.com/ and see if your computer/internet is enough to run a server. If it's not, you can rent one, like ServerCraft and many others, depending on your budget/need.
If you can run it, download the server files from here: http://www.minecraft.net/download.jsp
Then you have to open the ports in your router/modem, default minecraft port is 25565. (If you have a modem behind a router sometimes you have to do this: open the port 25565 in the modem setup to the router IP that the modem recognizes, then, open the port 25565 in the router setup to the computer local IP)
After that, run server.exe and wait for it :)
Run Minecraft, multiplayer, join localhost:25565
Give your friends your IP and port, tell them to connect to yourip:25565 and voila :D
If you want an fixed server name, you can get one from no-ip.org for free, download the software, and it will be even easier for others to connect to you
More info: http://www.minecraftwiki.net/wiki/Server