and after the first try today with propellors I found out that I really need to stabilise everything but don't know how or where to start.
Welcome aboard. You didn't say what the exact problem is. So that is the place to start. I realize that you want the model to be more stable, but some details about what you are experiencing would help the experts here.
1. Your build choices look fine. However, I couldn't find any MWC user comments about the FLY-20A. Be sure to confirm it is rated for the 500Hz PWM update rate used by MWC.
2. The default PID's should allow you to hover. If the model oscillates in flight then reduce the PID values a tiny bit until the oscillation stops.
3. Some folding frames have poor rigidity and the motors can torque along the arm axis, which affects flight performance and PID tuning. So grasp each motor and try to rotate/twist the arm. The arm should be sufficiently rigid and not allow unnecessary motor torquing/rotation.
4. The props size is appropriate. I expect that a watt meter will show about 450 watts at full throttle with them. This is a good sanity check, so measure electrical parameters if possible.
5. Balance the props. Do not skip this step! Your local (or online) hobby shop will have cheap tools that are designed for this task.
6. Confirm that the convex side of the props is on top. If reversed, the props will have less thrust (no, the air flow will not be reversed if the props are "upside down").
7. Be sure to calibrate the ESC's. There are endless discussions about how to do this.
8. Do not mount the sensors rigid on the Arduino shield. They should be mounted using self adhesive servo mounting tape or equivalent. The wires connecting to the sensors should be flexible, short, and very lightweight gauge (to reduce vibration issues).
9. Read the MWC FAQ and all the related links Again and follow the instructions provided. Read the FAQ many times until your eyes bleed.
10. Your model may be fine; It may be untrained fingers that need more time at the sticks. What I mean is that, despite how easy a Quad looks to fly, and all the endless videos on YouTube that seem to confirm they fly themselves, consider it all an illusion. It looks easy because the good pilots practiced for many hours.