Put together a Joker-like attire. He may look like a clown, but the Joker actually dresses quite nice. Wear a dark colored dress shirt with a brown tie and matching vest. Put on a matching suit jack over that.
Add in a trench coat to help complete the costume if you are going to be going outside in the costume. If not, then the suit alone is perfectly fine.
Purchase a pair of cowboy or dress boots. While they are not a big focus of the costume, you will want shoes that can help match and complement the suit.
Wear a pair of purple gloves. Purple has been one of the signature colors featured with the Joker and the tradition continues with Batman: The Dark Knight. The gloves will definitely help you stand out as the Joker.
Apply a layer of white face paint to your face. This will represent the base of the Joker’s face paint. Use a couple of extra layers to make sure that the paint looks like a pure white and no skin is showing. Even out the white paint so it almost looks like natural skin.
Use black face paint to create a large black circle around your eyes. If you can force some tears out so that the paint spreads naturally with water, then the face of the Joker can appear more distressed and lifelike.
Use red face paint to paint your lips. The Joker’s face is deformed so the lips need to extend out on the sides. To achieve this, just extend the red face paint to about the middle of your cheek. Use pictures as a reference.
Grow your hair out a little long, or use a wig to recreate the Joker’s hairstyle. Comb the hair down so that it looks stylish or mess the hair up to show that the Joker is in the middle of a battle.
Complete the costume with a fake weapon or prop that the Joker would be seen with, like a knife or a gun.