There are three things to consider in determining if clothing is Halal – coverage, material, and purchase.
Coverage is pretty simple. A man must cover the area between the navel and the knees. A woman must cover everything except her hands, face, and hair. I am of the opinion that a woman only needs to cover her hair in the masjid, but there is difference of opinion there. Clothing that does not properly cover the body is haram.
Some materials are haram. I am of the opinion that pigskin is haram, whether tanned or not. Also silk shirts, pants, robes, and coats are Haram, but only for men. Women may wear silk. Gold is haram, but only for men.
Finally, one must purchase clothing with Halal money. One may not steal clothing or steal money to use to buy clothing. One should acquire money by working or by gifts or by investment and the like. That money can be used to buy clothing.
Muslim Attire 