It may seem odd that a chair would be listed as a key component of your trading equipment, but if you consider that you will be sitting in it for 8 to 10 hours a day, you realize that it is very important. If you are not comfortable when trading, it will diminish your performance. Therefore, you should try to use a chair that you find comfortable. It should have an adjustable seat height, an adjustable back with lumbar support and an adjustable seat plane (angle of the seat that you are actually sitting on). Also the cushion should be comfortable.
You don't have to spend a lot of money on your chair, although you generally get what you pay for. The most important thing is that the chair is comfortable, even after you have been sitting in it for long periods.