If you have an interface that takes XLR, then it’s best to avoid getting USB mic.
Do you have a microphone stand already?
My last question is if you have any sort of audio/music shop in your city? Guitar Center, for example. Many places will let you try out and test different microphones before you buy them.
A simple cardiod, condenser type microphone may be the best place to start looking.
Here’s one that I’ve seen recommended a fair amount: http://www.sweetwater.com/store/detail/NT5Sing