I need an external microphone! But which one?

So i have a youtube channel and i want to go further than where i'm already!

i want to get a microphone becasue of the nasty echo in my recent video blogs have been making me upset! I have a Nikon D90 that i received as a gift. I don't want to spend more than a MAXIMUM of $150 for a quality mic! I'm still a college student so i can't be spending too much!

I did a career in television news / sports photography and audio. I always had very good luck with
Shure and Sennheizer.

And you know the D90 does NOT have a mic jack. So you will be using the quality mic post production?