I am a golfer and I have been trying to do a speed radar, similar to those that cops use. The problem is I have no idea which sensor should I use. Can anyone help? Appreciate it.
These are the 'sensor' components available in the App Inventor tool box Sensor components. Unless you intend to attach an Android to the golf club head and measure the acceleration; none of these are going to work for you Henrique. The sensors are inappropriate 
for determining, sorry. The AccelerometerSensor might be able to measure the impact but might damage your Android. If you did that you would have to determine the club's speed based on the shock recorded when striking the ball. This method of course is impractical.
You might be able to use a radar sensor to measure the ball's speed (you would have to use a sensor in conjunction with an Arduino or something similar) or the speed of the club's head . You might be able to use an Arduino radar gadget connected to your Android with Bluetooth to measure the ball's or club's velocity.
Someone else might have other ideas. Sorry, interesting idea but probably not practical.