Assuming you already know how to write the articles, I would then go to duotrope.com. It's a free website dedicated to matching up writers with a buying market.
I wrote up a hub about this just last night. Kinda odd, eh?
As for what to write about ... write about whatever it is you want to. There's a market for almost everything out there ... you just need to find it. The trick is to market your articles to the magazines most likely to publish them. You wouldn't send recipes to a sci-fi mag any more than you would send car reviews to a cooking magazine, but the nuances go deeper than that ... By reading the magazines you want to submit to, you get a strong idea of what they are looking for and you can tailor your article to sell.
The most important thing you must know is that you can't force an article to sell. If it's not right for a certain magazine, you can either make it right, or you can try to fit it in elsewhere. Trust me ... after ten years of selling short stories, I have learned to rewrite to meet the need, as it's a much longer road to search endlessly for that one place where it might already be the perfect fit. ;)