@Fall asked
Depending on the language and the joystick axes desired, you can just do roughly
motor[motorPort]=vexRT[Ch2]/2;
or you can do
motor[motorPort]=vexRT[Ch2]*.5; // in this you are multiplying and you can do percent so if you wanted 40 percent of speed it would be "*.4" and so on
The answers above are both correct. However, if at all possible, I would recommend halfing your speed through gearing or something similar. In these cases, you aren’t really changing the speed, you’re changing the power. While this will slow down your robot, it will also reduce your torque. Alternatively, using a torque gear ratio would half your speed and double your torque