Should I buy an audio interface to support my AT2020USB mic? Is it
necessary to buy it...
If you keep the existing microphone, than no. The primary purpose of an audio interface is to convert an analog signal into a digital one. Because your microphone is a USB mic (and USB cables carry a digital signal) there is already an analog-to-digital converter built into the mic. No further conversion is necessary or possible.
Will it sound better with an audio interface?
No, your USB mic will not work with an audio interface, for the reason you mention below.
As I mentioned, I am using an AT2020USB mic, how do I plug my USB mic
into the interface? I thought there is only XLR plug for the input?
You are correct. XLR cables carry an analog signal rather than a digital one, so they need to be converted to digital first, hence the need for an audio interface.
The benefit of the USB mic is that you have everything you need integrated into one convenient package, without the need for anything else. The downside is that you cannot incrementally customize or upgrade your toolchain. For example, I have a non-USB version of the AT2020 along with a cheap (M-Audio) interface, therefore, I could theoretically upgrade the interface without changing the mic (or vice versa). In fact, the audio interface is useful to me, because I can throw in a different mic (such as a dynamic one instead of a condenser, if I wanted a specific sound), or even plug in a guitar.
So, yes, an audio interface can provide an improved sound, and a greater flexibility. But it is not needed (and, in fact, not compatible) with your USB mic. The question you should be asking is: is your current mic sufficient for your needs?