My nikon d5000 says my subject is to dark, so when i take a pic it comes out black.why?

Also after i take a pic it gives me an error message

You need to read your cameras manual on how to read the exposure meter. I would suggest you take some basic camera courses to understand how ISO/shutter speed/aperture affect the exposure.

This is, I'm sure, a lighting issue. In those situations are you using flash or adding light some other way? If not, that's probably it.

I agree read the book. But, as stated, it's also important that you understand the relationship between ISO, shutter speed, aperture and light. This understanding will help you in full manual and learning how to use those settings should increase your quality.