I've written two articles for edencafe and have at least one more upcoming. write about something that you find interesting, research it and try to keep it at least partially current. First person is an option, when writing, as is third person, it's just a matter of preference.
establish most of your main points in the introduction, in order and restate them in the conclusion. make sure each paragraph flows into the next, as best as possible.
Don't try to fill space, it's painfully obvious. if you're going to argue for a position, don't be judgmental or arrogant regarding the other side, it lessens your argument. re-read it several times and read it aloud, that catches grammatical mistake the eye misses.
Also, I've written articles for my school papers for years and eden pays, by far, the most per word.