Which prime

I have had my Canon T2i DSLR for about 6 months.  I do like my 18-55 mm "kit" lens for versatility, and I use it most when I need wide angle shots at 18 mm, but also when I'm traveling.  But, wow, the photos I get from my Canon 50 mm f/1.8 prime lens are incredible!  For me, the photos I get from this lens is one of the main things that differentiates the DSLR from a "regular" camera in my opinion.  People ask me where I got the photos taken (i.e. which professional studio?), and I tell them I took them on my own.....

Regarding your question.............. I was very close to getting a Nikon D5100, and did a lot of research on their lenses.  I would recommend to get either the 35 mm f/1.8G AF-S DX lens, or the 50 mm f/1.8G AF-S FX.  They are both ~ $200.  The 50 mm lens would have a "more close up" view than the 35 mm.  The 35 mm would have "farther away" view, and might be more versatile.  I personally would try the 35 mm f/1.8 if I had the Nikon D5100.

I don't think the Nikon 50 mm f/1.8D "AF" lens, which is cheaper ~ $135, will autofocus with the Nikon D5100, so I would avoid that personally as a beginner.