Well, you'll want good frequency response range first and foremost and 20Hz - 20,000Hz is pretty much perfect, you'll be able to find some cheap stuff with 50Hz - 16,000Hz which is more than enough for Youtube stuff, and extra features like background noise cancellation is always nice if you don't expect to be in silence. A good sensitivity too. The best way is to just read reviews though to get an idea, and professional reviews at that, not just Amazon ones or something. If you want a very good microphone, bang for buck, can't beat this ol' monster:
Amazon.com: Blue Microphones Yeti USB Microphone - Silver Edition: Musical Instruments
Remember you'll need headphones if just using a microphone too, otherwise it'll pick-up your PC's sound. If you're looking for a full-on headset, anything in the Logitech G*** series is usually a safe bet, though just check out reviews and find something in the price range.
