Hi,

Find the diameter of the sprocket between the teeth, and in between the teeth. Make a cricle the diameter of the distance inbetween the teeth, then extrude that to the thickness you need. Then put a skecth on that, and selcet the contuction line option (top right corner in the drop down menu), and make another circle the diameter of the larger distance, from tooth end to tooth end. Then make sketch on that, from the inside circle, to the constuction circle. You will need to make some measurements on the angle of the teeth and such, but one you have that, finish the sketch, and extrude the single tooth to the thickness you need. Then do a circular pattern, picking the number of teeth in the sprocket or gear.

Hope that helps.