To create a nether portal you need 10 obsidian blocks. This can be quite a challenge - you need a diamond pickaxe to mine obsidian and obsidian is rare. Obsidian is made from lava and water, so look where you see the two things together to try to find it.
Once you have them, the blocks must be placed 5 blocks high and 4 blocks wide to create a frame. Note that the corners of this frame can be left out. Once this is done, the bottom of the portal must be lit on fire with flint and steel (a fire tool, crafted with 1 flint and 1 iron ingot), then the achievement is yours.
Refer to this video if you're having trouble:
All credit goes to the original writer.