How do I move my spawn point?
In Minecraft Beta 1.4 and later, sleeping in a bed will set your spawn point.
On older SMP servers running Minecraft Beta 1.3, spawn points can be changed using the /setspawn command, provided the server is running any of the common mods to provide this functionality. Sometimes this command is called /sethome, and sometimes the /sethome command has a different purpose.
There is no way to set the spawn point ingame on Single Player Minecraft Alpha/Beta < 1.4. Some third party tools will change it for you however.
For Minecraft Classic, pressing Enter sets the spawn point.