Which lense to buy for portraits? For nikon d3100?

I have nikon d3100 cam. 18-55 default lense. My portraits were appreciated and many times i've be invited by frnds to shoot their special days. Having a request for wedding shoot next week. Its just fun, not very professional but as i was getting many requests lately, i thought i should get a better lense (leave aside the cam thought for now). I could spend around 10-20000. Suggest me a lense for portraits. And any do's and don't tips while buying it?
(bonus: if you can suggest anything for betterment of my portrait shoots especially during wedding)

Back in 35mm film days, a short telephoto lens between 70mm and 90mm was reckoned to be best for portraits, often an 85mm lens.

On a DX camera like the D3100 the nearest equivalent angle of view would be the 50mm f1.8 AF-S lens. That is a good, very sharp lens with good bokeh and is great for portraits.

Make sure you get the AF-S version though. There's a cheaper, older AF-D 50mm f1.8 lens which will not autofocus on your D3000.