To create a nether portal in Minecraft, you will need at minimum 10 blocks of obsidian and some way to make a fire, the easiest way would be using a flint and steel.
 
Place the portal in like the diagram bellow, where X are any blocks and O are the obsidian blocks, and the portal is standing up vertically.

XOOX
O    O
O    O
O    O
XOOX
 
Then use a flint and steel on the inside ring of the portal and it will activate it.
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