OP, I gotta go with the consensus here, you really shouldn't try it out with $100.
First, because you'd be hard-pressed to find a broker without a minimum equity of something like $500 or $1000.
Second, because you'll get raped with fees. If your fees are even somewhat low, like TD's $7 per transaction, you'll still need a 7% gain to break even. Over the long term, the stock market is estimated to rise about that every year so you'd likely need about a year, just to get your money back. You'd need another year to be able to sell with an even $100.
That doesn't mean you can't start preparing for the day when you'll have enough for investing to really make sense though.
Here's some advice
(and you must be willing to put at least some time into this before you even begin to think about investing in individual stocks. Otherwise, skip to the bolded paragraph):
1)Visit Investopedia.com and just start reading about the basics so that you can get acquainted with the lingo of investing. You'll also learn about the different tools that help with investing (fundamental analysis, technical analysis and other techniques such as dollar-cost averaging)
2)Start following economic and financial news. That will inevitably help with number 1, allow you to make more informed decisions and help you with understanding the big picture.
3)Find companies you like and that hold potential. It may be something inherent to the company's business (for example, if you think everyone will have a dozen robots 10 years from now, you'd probably want to buy a robotics company) or some reason related to its stock (for example, a company with good future potential that falls 50% on a slight earnings miss, wouldn't that be a great opportunity?). In other words, you want to have a reason for buying a specific stock, you don't buy it just because you have money to spare.
4)If you want to spend more time learning about it, I suggest reading books, which will often offer much better insight than anything you can find online.
I suggest starting with Peter Lynch's books (Learn to Earn, One Up on Wall Street and Beating the Street in this order). All of them are very easy to read for the layman and offer some great advice for starting out.
If that seems like a lot of work and you don't want to spend this much time preparing for investing, then consider instead index investing, where you just put the money into a fund that tracks one of the indices, or just staying out of the stock market. The same warning about fees will apply, but your risk will be greatly reduced overall.
I thought you needed like $20000 to get into trading? I'm probably wrong.
Depends what you mean by trading.
If you're thinking day trading, that's much too low to really be profitable.
If you're thinking swing trading/trend following, then that's about right. You could perhaps make do with a fair amount less than that.
For buy and hold, a few thousand dollars will be enough since fees will be only one-time and you have a lot of time for your stocks to make you money.
