Before you start selling your services, make sure you're up to the job. By that I mean, get a few personal sites/apps under your belt first so you can prove to yourself that you understand everything that's involved in making a certain kind of site. You'll built a portfolio and it will improve your:
- Quotation ability -- you'll be more accurately able to judge the level of effort involved in a job, and help you decide on an hourly rate.
- Perceived customer value -- if you're able to offer suggestions to the customer, instead of simply checking off their list of functional requirements, then you'll end up with a happier customer.
Nothing will annoy a customer more, damaging your reputation in the process, than a half-assed job done by a complete newbie.
As for making $100/month, I know rails developers that are charging $100/hour.
In my opinion, as a new developer starting out, it shouldn't be about the money. Freelancing requires a lot of self-motivation; you're the boss. If you've not got a passion for web development, then you're going to hate your job. If, a couple of months down the line, you find our you're actually quite good at this Ruby/Rails lark, then the money will be a bonus.