In order to construct a Nether Portal you'll need Obsidian and you need 14 Blocks of it. You can create Obsidian by releasing Water into Lava or you can sometimes find it underground in a similar scenario. Once you have it you'll want to construct your Portal 4x5 so 4 along 5 up.
Then get flint which is made by destroying Gravel and combine that with an Iron Ingot and you have the tool to light up the Portal so walk up and use the flint and the Nether Portal will light up allowing you to cross into the Nether.
This is a video I made for my channel. It shows you what to do as well as the On A Rail Achievement.