Not all USB cables are suitable. To connect you iPad Pro, you need to ensure that it is a full data cable (some cables only have charging-pins connected) - and if you intend to use the fast-charging feature of the iPad Pro, it must also be PD (Power Delivery) compliant.
It has been observed that the iPad Pro series can be a bit “picky” about the cable. Apple cables, if available, are always likely to offer greatest compatibility - in particular for charging iPad Pro over Lightning.
If you have purchased a brand-new 2018 iPad Pro, you should have a compatible USB-C to USB-C PD cable. If you need to connect to a USB-A port on your PC, you may need to buy an additional cable. If you can, try to make sure that it supports the USB3 standard as this will provide greatest compatibility - and with a suitable PC, will give the greatest data transfer speeds.
At present, I’m not aware of Apple selling a USB-A to USB-C cable. Note also the PD is not available over USB-A (PD is not part of the USB-A specification, and as such, fast charging will not be available using USB-A cables).