A major step in Minecraft is building a portal that transports you from the classic overworld into the Nether dimension, which is a dangerous place with an interesting new set of blocks and monsters. To unlock more items and reach the next stage of the game, you __have to reach the Nether.
This video shows you how to build various sizes of nether portals. You create a nether portal using blocks of obsidian, which can be obtained by using a diamond pick axe. The smallest nether portal you can build requires only ten blocks of obsidian, but since diamonds are rare and difficult to find, it can take awhile to mine enough obsidian to create a nether portal. Alternatively, you can create obsidian by using lava and water, and this video shows you techniques to use to efficiently combine the two elements to create obsidian. Once you __have created your rectangle of obsidian, you turn it into a portal using flint and steel from an iron ingot to light the interior. This creates a purple smoke, which is the portal to the Nether.
Once the portal is created, you will stand inside the frame until the screen fades into purple. You will soon appear in the Nether next to a second portal that can take you home.