What you're looking for is a good shotgun mic. That in itself is a
generic term that means any long barreled mic. You want a good
condenser (externally powered) mic with a “lobar” pick up pattern
to put on your boom pole. Lobar pick up means the mic focuses
its audio pick up to a narrow area. This is why you want to use a
boom pole to get the mic as close the the actor as possible - you’ll
get clean dialogue tracks with less ambient noise.
http://www.sennheiserusa.com/newsite/
Sennheiser is the mic most used by professional sound recordists.
They are expensive and worth it. Check out the ME-66 and the
ME-67. The MKH60 is the best if you can afford it.
http://www.audio-technica.com/
Audio-Technica is cheaper. Check out the AT835B. It’s a workable
mic, a little muddy and not as directional as it should be. The
AT835ST is a good mic for the price, but it, too, sounds a bit muddy
- the vocals don’t sound crisp enough for me.
http://www.azdencorp.com/
I’ve never used Azden mic’s. They’re inexpensive and I don’t hear
too many good things about them, but if that's all you can afford
it's MUCH better than using the camera mic.
A good mic is an investment. It will last longer than your camera
- why skimp? A great place to check prices and purchase is B&H Photo
http://www.bhphotovideo.com/
Even when the audio on a camera is very good, you need to get
the mic close to the actors to record dialogue with little or no "noise".
The further away the mic is from the actors, the higher the volume
needs to be. The higher the volume, the more “noise” you get. Your
goal is to have a very high signal to noise ratio - more signal (the
dialogue) less noise (the background). Even a very good, expensive
Sennheiser mounted on the camera will pick up a lot of background
ambiance because it’s far away from the actors.
