Nikon D7000 best mic?

Hi, I recently bought a Nikon D7000 to make youtube videos with and I love the video quality. However, the audio isn't very good. I bought a Sennheiser mke400 shotgun microphone to go on top of my camera and it definitely picks up sound, but the audio keeps clipping all the time (the + is too loud and the - is too quiet) and gets distorted because I do voice impressions. I don't know what the best mic to get is. Are there any camera experts out there that can assist me? I would greatly appreciate it!

I'd bet your clipping is in the camera. I did a career in television news / sports photography and we always used Sennheiser mikes and never once did I have a mike clip - I'd bet it's an audio setting or perhaps it's automatic.

Perhaps your owner's manual will deal with that or you can call Nikon. Sennheiser makes some of the best products out there.

I agree with Jim. The Sennheiser has a range sensitivity switch but doesn't have any kind of decibel limitation (dB cut-off). You could experiment with the sensitivity switch if you haven't already done so.
Since this microphone has a standard 3.5 mm jack, you could test it by plugging it into the line-in socket on your computer and try to record a few samples (using freeware such as Audacity) with it at various distances and sound volumes.
If the clipping still occurs in your computer audio recordings there might be something wrong with your mike and you should have it fixed. If not, check your camera settings in the Movie Settings menu, as Jim has already suggested.