[onion2k]

Talk to people. Go to meet-ups, networking events, cold call potential customers, get on social media, use free promotion services. Google 'growth hacking', which is pretty much the art of building traction at minimal cost. Find a cofounder who is well-connected and get them to advocate your product. Find evangelists who'll shout about your product simply because they love it. Hustle.

And remember that for most technical founders, the marketing side of startups is really damn hard compared to writing code, and is absolutely the main reason why most startups fail. You really should have this stuff figured out before you write a single line of code because without it you have no chance of succeeding. Products do not sell themselves.