There are two different solutions, printing to the terminal or using the SD Card. If you have your controller or brain connected your computer and are using VexCode you can print to the terminal.
Ex. std::cout << "Velocity1 = " << x << std::endl
The other is to store data to a file on a micro SD Card. This can be done using basic C or C++ File streams. Below are a few different links to useful resources.
SD Card Help
V5 Robotics Brain SD CARD?!?
Vexcode printf error