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.
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.
An underwater world seed for Minecraft is used to open an underwater world to play the game in.
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
there is no seed for planetoids you have to get an external world generator to make it
-3085574207588475592
World seed-404
Well there is one way to get into good worlds in Minecraft. Type in a seed.
Every Minecraft seed is capable of producing dungeons. Seed 19981012 is a known seed that spawns you directly next a dungeon on the surface. Some seeds have more dungeons then others, you will just have to hope that you randomly get a seed that has a lot of dungeons.
sand cave
Nyan
The flat world seed spawns alot of slimes. But every seed spawn slimes.