dig down into a cavern, and find a place where lava and water are hitting. Then under the water there will be obsidian. Get it with a diamond pickaxe, and get a minimum of 10 obsidian. Then you can make your portal. Once the portal is made, take flint and steel and light it and step inside, your screen will get very wavy and wait and it will say, "entering the nether" and there will be lots of netherrack. This is very useful because the top of it lights on fire, so you can put netherrack that is on fire on top of mountains leading back to your home, so if you get lost, you can find your way back.