How to improve my skill at writing a question?

I'm a member of some SE sites, like SO and CSTheory. In those sites I'm an active member, I'm reading Q&A and also answering some questions or asking questions, but I think my writing is poor.

I like to write letters without thinking about grammar (like a child), but I think I'm a bad writer, because too many times my questions and answers are being edited. In fact, most of my problems are in my questions. I don't know how to ask good questions, which means I can't express my problem well (but my answers are not too bad).

I want to improve my writing, specifically when I want to ask long questions relating to my field (Computer Science). What's a good practice for this?

Answer

I commend you for wanting to improve your writing. :) You can look at how other people have edited your questions and compare the changes. For example, in your question above:

I'm a member of some SE sites, like SO and CSTheory.

You need "and" between two items in a list. You need to end a sentence with a period.

In those sites I'm an active member.

Indefinite article before the noun. You then have a string of clauses which should be separated into sentences.

I'm reading Q&A and also answering some Q or asking Q, but I think my writing is very poor.

Join the two clauses with "and."

I like to write letters without thinking about grammar (like a child), but I think I'm a very bad writer, because too many times my questions and answers are being edited.

You've written many posts (letters), so make that plural. You are referring to "child" and "writer" as generics, so use the indefinite article. Change the tense so it's not happening in the future, but describing a generic event which happens repeatedly.

You get the idea. (I also changed the title of your question because you are asking about how to improve the writing of any question, not just technical ones.)

Other than that, read a lot and study how sentences are constructed.