If you plan using PX4 for flying aircraft the most important question you should answer is, what will be the application. Is it for fun or for work, what is the planned flight times and coverage.
- If you need precision hovering and you don't mind shorter flight times then focus on Multicopters.
- For longer flights and larger areas of coverage you will need fixed wing aircraft i.e. Airplanes.
- There is even mixed type of aircraft called VTOL - Vertical Takeoff and Landing aircraft. It can take off in vertical mode like Multicopter and then transition in forward flight like Airplane.
The airframe reference lists frames that have been tuned for perfect flight with PX4.
Once you select and buy or assemble your air-frame you will have to preset it in QGroundControl with a template during the Initial Setup process.