I don't think there is a straight answer to that, however there are couple of things you could do to as a quick start:
- Submit it to application directories. For example, if you are making a JS thing you can submit to the popular JS websites. E.g: Echo.js, Reddit, etc.
- Make it easy for developers to take your project for a spin.
- Tell everyone about it at meetups.
- Find influencers and tell them about what you've done. They might be able to help you spread the word about it.
- MAKE IT THE BEST!
Good luck!