Is it ethical to share the knowledge for free that I've learned at the university?
I do not see that sharing your knowledge would in any way be a problem per se. What could become a problem is if you also share copyright-protected materials. It is virtually impossible to list what might or might not be such materials but to take other persons presentations, images, data and then sharing it would be clearly illegal (and unethical) unless they are provided with a "license" stating they are free. If you take the knowledge you gather and then put it together somehow (including making your own presentations on your own material), it should not be such a problem. In any circumstance where you want to use other peoples materials, it is always best to ask for permission. Not only does it save you possible future problems, you may find friends in the process. Watch out for materials published by commercial interests and use open source material (but do give credit to those who made it - attribution is required by licenses like CC-BY-SA and it is also a nice gesture to acknowledge the work and time by the original authors). Much material is given out for public (but not commercial) use. 

A final advise, attach an open source license to your materials. I am not fully aware what licenses may apply but am sure many has good suggestions for you (check Academia.sx or ask another question on that).

So to sum up. I think it is a nice idea and perfectly fine, but be aware that you must be 100% sure you do not publish materials so that you break copyrights or abuse licenses (protect the open source practises).