It all depends on what you want to do. If you want to get started cheaply the kit lens is more than enough.
Prime lenses will have larger apertures that can give you more creative options.
How close do you want to stand? Indoor/outdoors? What are you planning on taking pictures of? D7100 is APS-C, I find that 50mm (~75mm ff) on APS-C doesn't give you quite enough room indoors to take photos. So you might want a 35mm prime or a zoom that goes down there. If you're planning on taking portraits you don't want something too wide (~20mm and below can be good for real estate/architecture) because it makes people look weird.
Most everything else is dictated by how much you want to spend and how large/heavy you want your camera to be.
Personally I have a 35mm f1.8 on my camera and am happy with it, I use it for family outings, a lot of portrait-level shots and just general "hey we're at the museum" kind of photos.
I have a D7100 as well and a 35mm 1.8 and 20mm. Both are great. 35mm on APA-C is about 50mm on full frame and is the "natural" view. Generally too narrow for landscapes and streetscapes, the 20mm starts to be good for those.
Prime lenses will have larger apertures that can give you more creative options.
How close do you want to stand? Indoor/outdoors? What are you planning on taking pictures of? D7100 is APS-C, I find that 50mm (~75mm ff) on APS-C doesn't give you quite enough room indoors to take photos. So you might want a 35mm prime or a zoom that goes down there. If you're planning on taking portraits you don't want something too wide (~20mm and below can be good for real estate/architecture) because it makes people look weird.
Most everything else is dictated by how much you want to spend and how large/heavy you want your camera to be.
Personally I have a 35mm f1.8 on my camera and am happy with it, I use it for family outings, a lot of portrait-level shots and just general "hey we're at the museum" kind of photos.