Ok so i have a Nikon Dsomethingorother that i've forgotten as im sat at work

...anyway...i've only got the kit lens which is great, but, just wondering if there is one lens that i should get for photographing people (mainly in a candid manner) what should it be and why?
My apologies for my appalling memory at what camera i have....but my memory in general sucks suprised ive still got a job....
