How do you make a portal in Minecraft?
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.
Can I make it bigger?
As of snapshot 13w37a, you can now create portals of any size between 2x3 and 21x21. This is a feature in Minecraft 1.7.