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.
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It is not a seed, it is a world download. Look up dropper world download on google or minecraft forums
If cheats are enabled in your world, do /seed to find your seed.
the seed 1234 will give you village with blacksmith and a stronghold at spawn
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Well there is one way to get into good worlds in Minecraft. Type in a seed.