You may only build an ender portal with the use of creative mode in version 12w23a and above (when you download the game it is on version 1.2.5 and must be updated manually to get 12w23a or above snapshots) or by the use of third-party tools/mods.
In creating a world (not classic minecraft) the game creates a ender portals that randomly spawns once for each of the three randomly created strongholds.
They are created as follows:
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With the use of an 'Eye of Ender' you can travel the a place called 'The End' by placing one eye of ender in each block (right click). When the ender portal is spawned it may have some Eye of Ender already in it.
They are unbreakable in vanilla survival mode so you can't get the block.
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Answer in short:
Not as soon as you download minecraft, but with 12w23a or above then you can on creative mode.
It is able to get with use of a mod on any version as long as it was built for that version.
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There is no way to build an end portal except on creative mode, instead you must find a stronghold which contains a portal using eyes of ender. The portal must be activated with 12 eyes of ender before you can travel to the end.
You can create an Ender Portal using Ender Portal Frame blocks from your inventory and activate it using Eyes of Ender
Ender Portals are unbuildable in Singleplayer, but if you go into creative mode, you'll find the End Portal Frames and Ender Eyes in one of the tabs, the only way to find them in Singleplayer survival is to use Ender Eyes to follow and find the location of the Strongholds of the Portals.
You have to put 12 eyes of ender in each of the portal blocks. Then, it should activate.
1. You can't make one from scratch 2. You can find one in a stronghold. You need to put ender eyes in the outside of the portal slots.