There are different types of spoilers. These include a genuine spoiler which is designed to diffuse the air flow, and the other is a wing, which increases the amount the car is pressed down onto the road by the airflow, in other words the car’s downforce.
Downforce is created by making the air flow faster on one face of the spoiler than on the other. If the air flows faster on the lower surface, then more downforce is generated. You could think of it as an upside down aeroplane wing.
Ideally a wing type spoiler is located as high as possible on the car. The higher it is mounted the more efficient it is. This is because the air flow over a car is turbulent rather than laminar and wings do not behave well in turbulent air. Just think of what happens to an aeroplane when it encounters turbulence.
Even if the wing is correctly mounted and the air is not too turbulent, then it is only effective at relatively high speeds, say 60 mph or more. At these speeds its effect is to increase downforce so that the car has more grip when cornering, so if it is your intention to drive your car at high speed on winding roads, then you might see some improvements in how it handles.
A non-wing spoiler works by breaking up the airflow and reducing drag. This increases the efficiency of the car and at the same time it also has an effect on downforce by slowing down the speed of the air flowing over the top surface of the car which causes lift. Effectively it is a lift reducer rather than a downforce inducer.
Adding spoilers and wings to a car can improve the performance and handling if it is done correctly. It is important that their characteristics complement the existing aerodynamics of the car rather than impeding them.
If you are looking for a used to car modify to your own style it is better to buy a used car, the main reason for this is so that you do not interfere with the warranty because the slightest adjustment could void it.