Schwab is one of the many brokers that will do this for you. Vanguard is another very popular one, as is Fidelity. I would do your research first to see what the account minimums are at each broker and what fees they charge (any sales commission charges when you buy into a fund? Are fees waived when you set up automatic investing?). If you buy into an index fund, as Acorn has suggested, fees will be much lower than other mutual funds. I'm a fan of index funds.

Acorn has suggested buying into the Total Market Index (once again, Vanguard and other brokers offer their versions of the Total Market Index fund). Alternatively, if you want a little bit more customization you can try investing in three different index funds: Total Market Index (which invests in US stocks), Total International Stock Index (invests in international stocks, as the name implies), and the Total Bond Market Index (invests in bonds, which are low risk/low returns, but they do great when the market is going down). You can set your allocations, so it will split up your $100 between these three funds each month. Each of the three index funds has its pros and cons, so it does give a bit more customization. That being said, this post seems to be geared towards a very basic level. In that case, I do agree that a Total Market Index fund is the best way to go (once again, that invests in US stocks).

One more quick suggestion: I would highly recommend reading "The Elements of Investing" by Burton G. Malkiel. Really cheap book, very concise and very easy to read. The whole purpose of the book was to boil investing down to the absolute basics & must-know information. And to make it extremely easy to read. The book does an awesome job. Read it!