looking for motor advice

edit

Hi guys looking for advice on what motor to buy. I have a robot arm in gazebo simulation that I send random forces to and the arm learns to associate what force should be sent to be in a desired joint. So I need a motor that receives a voltage and outputs a position in radians or a degrees. I have been playing around with a small dc motor and power supply and noticed it goes faster the more voltage I send it, I now need to know what position the motor is in. My friend who knows about hardware told me I need an encoder and decoder, but I don't know how either of those would work with my motor. Can I get some product links from digikey or other online product retailers? thanks.