You need to be much more specific. Are you trying to connect a standard USB-A device like an external drive to a USB-C port on your new Mac? Or are you trying to connect an external device with a USB-C port to an older Mac? Or are you trying to connect an older Thunderbolt 1/2 device to a new Mac which only has USB-C ports?
It is also very important to know which protocol the external device uses since a USB-C connection does not tell us enough information. Some types of connections may be possible and others may not be depending on the protocol the external device supports. Does the external device support Thunderbolt protocol or the USB protocol? Sometimes an external device with a USB-C port may have an icon next to the port showing a USB symbol or a Thunderbolt symbol, but some devices may not show any icons so you will need to check the documentation for the device.
The newer 2016+ Macs which use USB-C ports allow for both USB and Thunderbolt protocols so you can connect older devices with the appropriate adapters.
External devices with USB-C ports usually only support one of the two protocols. An external drive will typically support the USB protocol, but other drives may only support the Thunderbolt protocol. This is especially important if you are trying to connect a USB-C device to an older Mac which uses the older style USB-A connector and Thunderbolt 1/2 ports (aka mini-display port connector). In this case you must match up Thunderbolt protocol with Thunderbolt port. USB protocol for USB port.
Unfortunately I'm having trouble locating a good article which can explain this a bit better.
