You can't plug a USB microphone into an Interface, because audio interfaces don't accept USB as an input. They're meant to be an all-in-one recording solution for improved quality on a budget. So technically you DO NOT need an audio interface with a USB mic.
Should I buy mic or audio interface?
An audio interface will allow you to record more than one thing at the same time. Depending on the number of inputs, you could potentially record multiple tracks in your DAW (Digital Audio Workstation). With a USB mic, it's the good at one thing scenario again. You can only record one track at a time.
Do you need an interface for a mic?
We've already mentioned that some mics have a USB output. If you only need to record one voice at a time, a USB mic will do what you need, and you likely don't need an interface at all. If you're fixed on a mic with an analog output, read on.