Yes, you enter it in the server properties text document.
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In singleplayer, when you start a new world, under the name of the world, there is a tab where you can enter a seed. If you are talking about a server, then edit the seed in the Server Properties file.
You drag it into the plugins folder. If you want plugins in your minecraft server use bukkit, not the vanilla minecraft server.
You can if you use mods or server texturepacks
Use the hamachi server.
Well there are many ways of setting up a minecraft server. Some people use the basic method to just use the Minecraft Server Application found on The website of Minecraft. But for an advanced server where you can use plugins you have to use CraftBukkit. To find it go on Bukkit.Org. For the IP for others to connect is a bit complicated. The easiest way which even i use is to download Hamachi. When you install it. Hamachi will give u a generated ip. Put that Ip in to ur Minecraft Server.properties. The IP there. Then you and your server friends have to use that IP to connect. And Hamachi has to be on for it to connect.