Invariant's answer is a good resource for how everything was started and what was the state of JavaFX on embedded and mobile in beginning of 2014. But, a lot has changed since then and the users who stumble on this thread do not get the updated information.
Most of my points are related to Invariant's answer, so I would suggest to go through it first.
UPDATE
JavaFXPorts has been deprecated. Gluon Mobile now uses GraalVM underneath. There are multiple advantages of using GraalVM. Please check this blogpost from Gluon. The IDE plugins have been updated to use Gluon Client plugins which leverages GraalVM to AOT compile applications for Android/iOS.
Old answer with JavaFXPorts
Some bad news first:
Now, some good news:
If you are not the DIY kind, I would suggest to install the IDE plugin on your favourite IDE and get started.
Most of the documentation on how to get started can be found here and some of the samples can be found here.