Drag is caused by air resistance and water resistance as an object moves through a fluid.
Drag ties in with resultant forces. You will need to take into account the size of any air resistance or water resistance when working out the resultant force acting on an object. From the resultant, you will then be able to work out the size of the acceleration on the object if you know its mass, using Newton's second law.
Force = mass x acceleration
(N) (kg) (m/s2)
As the objects speed increases, the drag will increase.
To decrease the drag, the object should be made to be streamlined. This generally involves making the object smooth, and pointed, so that fluids (gases and liquids) can easily flow over the object as the object moves through the fluid.