Since you're a developer, you already have the most important asset, which is the ability to basically create anything (web services related, obviously). From there, it's "just" a matter of finding something to build that other people will value and then telling as many people as possible about it.

The best way to figure out what to build is by thinking about what YOU would find useful, and then build that. Don't get too big with the idea, though. Just analyze your day-to-day routine and ask yourself what kind of little piece of software would make your day 1%-5% less annoying.

For example, I'm a developer but also a startup founder. I've wasted entire days doing repetitive email follow-ups to investors, partners, customers, etc., which means I wasn't committing code. So I put aside a weekend and built a system to automate my email follow-ups. After it worked well for me, I showed it to some colleagues, they started using it, too, and before I knew it, I had a nice little SaaS app going. With another weekend of work, I added a frontend and billing system, and I launched it as https://autopest.com.

(I'm including the link at the suggestion of some of the other folks in this thread, and also to show how it matches well with their advice -- target B2B, build a SaaS app, keep it simple, rely on quick solutions like Bootstrap and Stripe, etc.)

Step two is getting people to it. Best way I found for that is social media -- especially Twitter. It only takes me 15 minutes a day to be "active" on Twitter, I can easily target BizDev people and GrowthHackers (my target audience), and slowly but surely, they start signing up. It's been a few months and I'm on pace to hit your $500/mo target in the next 30 days.

Best of all, because I built something that I REALLY WANTED, even if no one ever pays me another penny, I'll still come out ahead because the thing I built works really well for me.