Once you've written your plugin, you'll want to get it published to the Play Store - just like any other app.
If you don't already have a Google Play developer account, you can find out more about how to sign up for one here:
http://developer.android.com/distribute/googleplay/publish/register.html
If you would like your plugin included in the plugin list within JuiceSSH you will need to email us the following information to support@sonelli.com:
{
"package": "com.sonelli.juicessh.pluginexample",
"description": "An open source example plugin which demonstrates some of the cool things plugins can do.",
"icon": "http://juicessh.com/images/logo-pluginexample-128x128.png",
"author": "Sonelli Ltd",
"email": "support@sonelli.com",
"url": "http://juicessh.com",
"opensource": true,
"repository": "http://github.com/Sonelli/juicessh-pluginexample"
}