You need a lot, but it also depends on what you are willing to invest. $100 can be enough for some areas, whereas $100k are bargain.
$100 is surely not enough to start with, I mean, seriously start, but to play with maybe it's enough, it depends on what we can spare.
I cannot be sure if there are investments that low for starting if you are trading. It would depend on the company that you trade with. I think that with trading though is a tough challenge that you have to know what your doing with your investments or you would lose all the time.
There are some companies that let you invest as little as $100. I don't know how that works and quite obviously you won't make much but assuming that you'll be doing this to learn the ropes, so to say, it wouldn't be such a bad idea to start off with such a little amount then move on to investing more once you're convinced that you can make money with stock-trading. Anyone interested just click on the link below to learn more: https://www.mint.com/blog/investing/how-to-invest-100-1000-or-10000-0114/