Primary lens for Nikon D5100

Hi all,

So I have been enjoying my camera with an 18-200mm lens, but I always hear people talking about primary lenses, and I could use a lower f stop lens. I wanted to know what a good lens to get is, with a f/1.8 aperture. I don't know which focal length will be the best, but since I don't have a full frame sensor won't a 35mm lens look like 50mm? Any recommendations are welcome, and thanks for helping!