There are really two skill sets you need. First, is a good understanding (both physical and theoretical) of Judo. Second, is a good understand of teaching physical activities.
For Judo specific knowledge, getting through the grades and doing some competition is essential. After all, first dan is knowing the basics; second dan is being able to do the basics; third dan is understanding the basics; and fourth dan is being able to teach the basics. However, you should be able to assist teaching at first/second dan and do your own classes at third/fourth. Of course, nothing whatsoever stops you from teaching earlier. Most organisation can certainly train assistant and full instructors at whatever rank they chose to. I have seen great teachers that were shodan and garbage ones that were rokudan.
Going competitions is essential as it will not only improve your understanding of kata but will give you experience of using your skills against resisting opponents. This is essential in understanding how to apply kata. Clearly, you should be able to perform the techniques adequately for your grade, although being a competition winner is not required provided that you did do some competition.
Teaching physical activities is a vast topic. There are whole university courses for this and while they are useful unless you wanted to become a physical activities coach professionally you do not need that level of details. However, you need to know how to coach people and I am sure Switzerland's Judo association will be running course for that. I would check those out in the first place.
In addition, you will need a first aid certificate. You should get one anyway. Everyone should get one. 'nuff said.
If you teach children (aka under 18), there are a myriad of regulations you need to follow. Again, your parent organisation should know all that. The law is quiet complex in that respect. You can get into a heap of trouble there without even realising you are.
Finally, since you are 15, I would strongly recommend learning and practising as much as you can. In ten years, you will be a third dan and in a perfect position to start your own club. In the mean time, ask your instructors what you can do to help. I am sure they will appreciate the help!