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/SKYVATOR1. 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
AND
QUICK SOLUTION*** Spoiler - click to reveal ***1. Go into a friend's world who has a Hellevator built and fly to the top using wings.
2. Build a giant tower of blocks (dirt, sand, stone, wood, or platforms under yourself by going into cursor mode. Then place the cursor under yourself and jump up while holding the RT button. The disadvantage of this is that if you get hit by an enemy, you will be knocked back and may fall off your tower.
3. Go outside to an open area (no ceilings) and drink a gravitation potion. Then reverse gravity by pressing UP on the left thumbstick. You will "fall" all the way up to the top of the world (and die if you are not protected from fall damage), but it will get you the achievement.