I am going to graduate this year, I want to apply to a graduate school overseas. But as I am not used to writing letters in English, not to mention to write to a profressor, would anybody please tell me how to write a letter to a professor?

Letter writing an art. I suggest you should start with Semi officially way. Mean you can write Dear Mr. XYZ, College address and your contain of latter.
Remember to explain about yourself, branch, period on which you want to join... etc.
At last you can ask some question for clarification llike procedure, fee, hostel accommodation.. etc.
At end provide your all contact detail.
Hope it will work out.

Thank you very much.

How to star to write professoional letter

First of all no slangs at all.u can use jargons as it is a professional letter n m assuming of the same field.
B polite but only up to a definite level.

I am going to apply in a overseas university for master( research /study) of civil engineering in space structures course and I would like to ask a famouse prof. who works on this special matter in that university if he admit me or not.
How can I write a lettet for him?