Q: My iPhone 6 seems to always use wifi sync when I plug it in to my computer instead of using the usb. Is there a way to fix this?

Check the settings in iTunes. There is a setting on the Summary page regarding using Wi-Fi to sync. You need to turn that off.

Since the Wi-Fi setting is determined in iTunes, I'm not aware that you can do a USB sync unless you turn it off, then you would have to turn it back on again. However, see this support document. The bottom talks about using the USB. I'm going to read and see what changes you can make. Sync your iPhone, iPad and iPod touch with iTunes using Wi-Fi - Apple Support

It does not appear that doing both is possible. But here is the other support document that it was referring o. Sync your iPhone, iPad, and iPod with iTunes using USB - Apple Support