Create apps! This is the most needed. We created a tutorial for Kirigami here: https://develop.kde.org/docs/kirigami/. There are also a few tutorials for GTK/libhandy with e.g. https://tuxphones.com/tutorial-developing-responsive-linux-s...
Also join the developer channels on matrix: #plasmamobile:kde.org (not sure that the address for the GNOME one)