What lens shoud I buy?

I had my first Nikon Dslr for over a year now. I mainly shoot portraits and now that I have better skills in photography, I think it's time for me to invest in new lens. I currently use my kit lens 18-55. What lens should I buy to get better image quality and bokeh?

For portraits buy the Nikon AF-S 50mm f1.4 if you can afford it and if not the AF-S 50mm f1.8 will do the job.

You don't say what model of DSLR you have. I'm guessing it's a DX (i.e.crop) model. In which case, the previous recommendation of a 50mm is pretty good. If you happen to have an FX (full frame) camera, then an 85mm (1.8 or 1.4) is going to be a good portrait lens, or (if you can find it) the 105mm F2 DC will be absolutely superb for portraits, because it's a lens designed for excellent bokeh and subject separation.