Equivalent of a Nikkor 50mm 1.8G AF-S?

I would like to know what would be an equivalent of a Nikon Nikkor lens 50mm 1.8 AF-S in Sigma lens? I would like to take good portraits and i have Nikon D3000. Thanks in advance!
Note: i'm starter in DSLR's.

Sigma makes a 50mm f1.4 and that's as close as you can get.

With the crop factor on the D3000 I would recommend you get a 35mm instead of a 50mm. 50 is an awkward length on a cropped sensor DSLR, but 35 is pretty close to what a 50 is on a full frame or a film camera - that nice length that's neither wide angle nor telephoto. I would recommend the Nikon 35mm f/1.8G AF-S DX Lens if that's what you're going for. It's on Amazon for $199 you can't beat that.

However, you say you want to take portraits and a longer lens is better for portraits due to the lens compression which has a flattering effect on your subject. I would suggest at least 75mm but you'll have to drop a little more money on it.