Many sellers on Amazon sell cables that connect a DMX light to a computer via USB. Generally, these sellers are using a popular FTDI chip to convert USB serial data to RS485, the standard implemented with DMX.
If you are looking into using one of these USB converter cables to control data (often in a single universe), here is what you should know first.
The FTDI chips often used require the installation of additional drivers. Make sure you match your cable IC chip to the drivers you download. These drivers can be found on the FTDI website - if you are on Windows, the easiest solution is to download the setup executable to install the drivers for you (look to the right of the operation system on the download page).
Once you have the drivers installed, you can use DMX control software (like FreeStyler) to add the fixture and send specific DMX channel data to it's address.
I encourage you to watch a YouTube video on setting up FreeStyler as it may not be very intuitive for those getting started (Define a fixture, add the fixture, control the fixture).