I've been struggling with a certain random number-related problem for a while. I need to be able to generate a random number from 0 to 100, weighting the 0 as most likely to occur and 100 least likely. When graphed it should look like half a bell curve, but the problem is that I don't quite know how to do it. I've tried reading up on bell curves, but I'm not exactly a great mathemetician.

The purpose of this random number generator is to decide upon a single monster in a large database of monsters which are given a rarity level of 0 for most common to 100 for extremely rare. Any help would be greatly appreciated.