Investing with No Money
If you don't have cash, you'll have to make up for it by investing your time. You can even find a partner or two that have the skills you don't (programming, sales, marketing, etc) that will also trade their time for a chance at hitting it big with a startup.
At some point you'll likely also need cash. Start talking to your friends and family. Tell them about your idea, tell them you need money (be specific to how much), and ask them if they're interested in investing or if they know anyone that might be. You might not know rich folks with $50k laying around. It's amazing, however, how many times pizza delivery friend Jamie has a rich aunt Gertrude.
Testing the Idea
Build a simple "coming soon" page that gives a brief synopsis of your idea. Open a Google Adwords account to drive people to it and see if you can find some combination of an ad headline, ad copy, and landing page content that gets people to give you their email address.
Talk to those people, along with your friends and family and see if they'd use your product. You'll also want to find other ways to meet people that might pay for your product or service and talk to them about it too.
If they Like It, Make It
If it's a physical product, you'll need to build physical prototypes. If it's software, you'll need to build a rough and dirty prototype. Either way, once you build it, you'll need to take it back to the people you've been talking to and have them try it.
If they Still Like It, Make More
Once you've got people excited about a rough version of your product, it's time to make a version polished enough to sell. If it's physical, you'll do a small production run. If software, you'll build a polished "minimum viable product" or MVP for short. An MVP is the smallest implementation of the idea that people will pay for (or use a lot if you plan on an ad revenue model).
And Don't Give Up
If at any of the above phases, you don't get strong positive responses from people, you'll need to make change and repeat that step until you do.
You'll also need to raise extra money between many of the steps. Most startups these days take hundreds of thousands to millions of dollars before they get good traction, and often hundreds of millions before they start turning a profit.
It takes a decent idea, and a lot of hard work and cash to bring a new idea to life. Often the one thing that separates success from failure is your will and ability to stay the course and try one more thing.
