The Eye of Cthulhu is usually the first boss players encounter, and he may be tough if you aren't prepared. We are going to build an arena that will be the setting of your Eye battles.

First choose a location: in the air, underground, or on the surface. In this guide I will be building one in the air, so monsters can't hurt us while we fight.

Step One: Build the outer wall
Build a large enough box so that you can maneuver around easily. this one is about 50x40

Step Two: Platformer
Next we will build platforms so we can move vertically. The number of rows of platforms is up to you, but less means you can move quicker. Here they are spaced four tiles apart

Step Three: Life Regen
Now add a cozy campfire, this will provide light and will grant you regen while in the arena. Our basic arena is finished. wait until dusk and use a suspicious eye and fight the Eye of Cthulhu

This is my personal Cthulhu arena:
It uses all of the things we talked about in this guide, and then some. It has demon eye banners, obtained from killing 50 demon eyes, that give nearby players a bonus against the demon eyes that Cthulhu's Eye spawns. The trophies have a 10% drop rate from the Eye of Cthulhu, and the treasure bags in the item frames are dropped from him when in expert mode.