Touching the top of the world requires you to build up to the top horizontal edge of the world (the opposite of Rock Bottom achievement). For beginning players, you will need to build up using blocks or platforms. The best way to do this is to create a Hellevator close to your home base and build an upper part reaching into the sky. You can use the materials you remove making the Hellevator to build the upper portion if you are early on the game.MAKING A HELLEVATOR/SKYVATOR
1. A fast pickaxe is recommended since this will require a lot of digging.
2. Make a hole leading straight down, at least two blocks wide (I recommend five because you can add walls later and line it with material to keep enemies out).
3. Place platforms across at intervals to prevent falling damage, or bring some buckets of water and drop them at the bottom to make a little safe landing area.
4. Add some light sources.
5. You can make branches out to the sides (and add doors if you want) to access underground areas for mining ore or exploring.
6. A Hellevator prevents you from being hit by making a wall on either side of you. This allows you to go up and down without being hassled by enemies.
This is also useful for going for