Thank you for all your replies, they are very interesting and thanks for the link Sarah!!I hope you can share more of your teaching knowledge and expereicnces!!

And thank you Kerry for your idea, it it good for me as I live next to a school.
Today I told my student's mom that I was going to raise my fee from 16 USD to around 18.5 uSD(I had not raised that for 2 whole years); she was not happy at all!! but I have to go to her house that is about half an hour away and 1 USD for the bus so I think it's fair, and she's wealthy, and I give her all the books and material.
Also I agree that it is only fair to start charging small quantities but then increase as you gain experience and with the years, here the best teachers charge between 37 and 45 USD per hour.