Fill it with old rags or old t-shirts...basically cloth. Mushinmaster was right on that. If you have one of the normal 3 or 4 foot bags, 60 to 100 pounds is fine. A 6 foot bag can be anywhere from 120 to 200+. If you want to do pure boxing, then wrapping your hands is very important. You will be able to hit harder because your wrist will be supported and able to handle more of a load. It is fine for your wrists and knuckles to hurt in the beginning. They will become conditioned over time. Get a video on proper hand wrapping, or ask a trainer to show you how. Just anything won't do.
Have fun and train hard.
