Which NIKON DSLR I should invest on?

I'm looking to invest and get my first Nikon DSLR camera, and confused which one to get, since I never used a DSLR camera, but like to learn on it.
between the D90, D300s, D3100 or D7000 which one I should get.

I would suggest you not go as high as the D7000 for your first time. It may prove to be too much camera for you.

The D90 is great, nearly perfect in all ways. The D3100 is the best Beginner DSLR camera that would probably be your best choice to learn on and continue shooting on for 3 years +

it really matters on your budget as well. The D3100 is the cheapest followed by the D90, then the D7000 and then the D300sg

Nice get a D300s and give it to me when you get fustrated because its too advanced for a noob like you.

Of the cropped sensor cameras, the D7000 certainly has the most features and is a robust camera.

If you are buying a camera to supplement your full frame D3, then the D300s would be your choice for a camera that can be used under the stress of of professional assignments.

Only you know which will supply you with the tool(s) you need for your projects

All are great, but the D7000 is too expensive and too advanced for a beginner that can't spend as much as he wants.

D3100 is great cause it's light and small, has less noise than all except for the D7000, better video and is great. It has no AF motor, so not all lenses will auto focus.

D90 - awesome, great build, fast AF and frames per second.

D300s - Just a D300 with video

Here's a post called 'Buying a DSLR, what's important, megapixels, features, brand, price, quality and which one to buy - http://www.the-dslr-photographer.com/2009/03/which-dslr-to-buy.html

The site has many more tutorials, tips, reviews and guides!