A Nether Portal requires at least 10 obsidian blocks in a 4x5 frame (corners are optional). Obsidian is created when water flows over lava. You'll need a diamond pickaxe to mine obsidian. Activate the portal by lighting the inside of the frame with flint and steel (1 iron ingot, 1 flint).
Minecraft
Crafting a Nether Portal — Minecraft Guide
Build and light a Minecraft Nether Portal using obsidian and flint and steel. Learn how to mine obsidian with a diamond pickaxe for your adventure.