Do I need a 18-55mm lens?

I'm a beginner but I'm fully acquainted with manual controls recently I have purchased a second hand Nikon D5100 with 50mm 1.8g. Do I still need to buy 18-55mm?

p.s: I basically shoot portraits & occasionaly landscape.

18-55 is the All-round lens for any Reflex camera, I used it for Landscapes, Portrait and Macro but the quality is about around 65-100 so it's not that bad.
It's a must have lens for beginners.