Ok, first decide what your server wants to be. Do you want it to be a huge server with a big community, or a small private server.
After youve decided, then decide whether you want to rent a server, or host it by your self. If you want to rent a server, shop around server hosts. There are many server hosts. I personally recommend DBK Servers. They provide a wide range of options, form a personal server, to a huge 100 person server. Their prices are the lowest out of most of the other hosts. Their customer support is also very good. I use it for my server.
If you want to host it yourself on your own computer, make sure that you have enough RAM and CPU power. Also, keep in mind your Internet connection.
After that, just decide what server program you want to use. Theres the default one that Notch has provided. There is also bukkit, which is like the copy of minecraft, except it provides plugins support.
Also keep in mind what plugins you want. If you want a large variety of plugins, use bukkit. They have many plugins.
If you have any more questions, just send me a pm. I run a server myself, so i understand how to run a server.
Here are the links to the services i mentioned.
http://www.dbkservers.comhttp://www.bukkit.org
