How does 3D printing over USB work?
This is just a stub answer. I will try to expand on it later.
The 8-bit microcontrollers used on many 3D printers do not have a USB interface, and so a USB to serial interface chip is used to allow a computer to communicate with the serial port (UART) on the microcontroller. In order for the computer to communicate with the printer, a device driver is needed to allow the operating system to communicate with the interface chip.
The appropriate device drivers should be supplied with your printer, and you should install these drivers before you try to do anything else. The drivers will make the printer look as if it has a serial interface, and the highest speed that most 8-bit microcontrollers can manage is 112800bps.
If the software that you are using has a terminal interface, you can send any G-code commands to the printer. For a list of G-code commands, see here. Note that this list includes commands for CNC machines as well as 3D printers.
For normal printing, G-code commands are sent to the printer using a print manager, such as Repetier-Host. Some slicers may have built-in print managers.