How can I get started with developing apps for Ubuntu Touch?

These links should get you started:

• Reference: Welcome to Ubuntu apps development!
• Sample App: Tutorials | Ubuntu Developer

How to run the app:
Once you get all of the SDK installed, you can connect your device via USB, QtCreator will be able to detect it, then press Ctrl+F12 to run it on the device.
Courtesy: mhall119

There is a 34 part QML Tutorial series by Joseph Mills on YouTube. You can download the tutorials using youtube-dl, which is a terminal app that you can install from the default Ubuntu repositories.