If you want to connect your iOS to PC via USB tethering then there are couple things that you need to do. First of all make sure that you have installed the latest version of iTunes for Windows. It can be found in the Windows Store.
Once you have iTunes installed go to your Phone settings and "personal hotspot". Tap the slider so you activate Hotspot and right beneath it you will see 3 different methods, one of them being USB tethering.
Now we just have to connect iPhone via USB cable to PC and select your device in the Network settings. While doing that, make sure that you have iTunes app running.
In general USB tethering is available straight away but some Mobile Carriers offer USB tethering as an additional service. If you don't have such service you will have to check with your operator.