No. You are the author no matter where your writing gets posted, printed, or said. Now, if you took my writing and posted it for revision without crediting me as the author of the original, that would be plagiarism. Also, if any of your work includes that from another author and you fail to cite that author correctly, you would be guilty of plagiarizing that author's work.
Plagiarism is a fancy way of saying "stealing." If you are not stealing, you are not plagiarizing. Beware, though, when putting something you learn into your own words that you
really put it in your own words. If your words too closely resemble another's, it is called paraphrasing and should be cited as such.
Hope this helps! It was an excellent question, but try not to be too paranoid. Just remember to give credit where credit is due and you will be fine.
Blue skies!
Jeannie
Jeannie Elliott