How do I get this fat bass synth sound. Please help out.
Thanks in advance.
http://soundcloud.com/viceroymusic/notorious-b-i-g-going-back-to

 

If you are describing the slap bass sound...I would recommend trying Trillian Bass Module found here....it has a ton of different bass sounds you can also edit to sound just like you want.
http://www.spectrasonics.net/products/trilian.php

 

Trillian is fucking ace. Best bass VST in my opinion when integrated with Omnisphere because of it's excellent synthesis capabilities as well as it's huge sample library. Make sure to pick up the Bob Moog expansions.

 

Sounds more like Logic Pro's Sculpture or another physical modeling (bass) synth.
As far as I remember, simple old Steinberg VB-1 sounds similar as well.

You can download VB-1 and Model-E (which is always good for fattening up VB-1 if necessary) for free on Steinberg web site (search for "VST Classics Vol. 1") for Mac and Win.

Of course you have to tweak VB-1 a bit until it sounds like the bass in your example, but that should be easy

Have fun.