As the comments above, the question is really broad, but hopefully here's a couple of points that can help. I'm a solution architect, and so I write very technical documents, I write semi-technical documents, I write business documents and I've written articles and blog postings. In the past I have also written product reviews for a magazine. I believe these points apply to all of those.
Q1. If I am writing a technical article on a software technology then is this called a white paper?
It depends, if you worked at the company (or you had invented it) and were explaining how it worked and were helping people wrap their heads around it, then yes. But if you say, had tried Xamarin and wrote about your thoughts regarding developing iPhone apps with it versus with Apple's Xcode, then that would be more of a product review. There are also editorials where people review, and discuss the technology and potentially what it means going forward, etc.There are also architecture documents, analysis documents and the list goes on and on. It really depends on your context and intent.
Q2. Is there any specific course(online preferred) which helps me in giving my writing skills a professional touch?
I'm sure there are, but I can't help on the exact path there. Would I generally recommend to people is not to over-educate themselves, and instead get into it and then get feedback. It can be easier to figure out what kind of course you need, when you come to understand your deficiencies and strengths, than trying to guess. Also, I have met a lot of people who hide behind courses instead of jumping you. You will never be "completely" prepared.
Q3. What are the rough points I need to down before I write an article? Are there sections that are required to be included? (For, I used to have software requirements and prerequisite knowledge as 2 mandatory sub sections.)
Articles is a very broad topic, so in terms of what sections, I can't help you there. It's too broad / too vague.
However, when I've written articles I always start with
1. Making points about the audience and what I want them to take away. I use this to remind me as I write who they are, their level of technical understanding, etc.
2. Figure out where you are trying to go, break down the key top level points. You shouldn't have too many of them, otherwise you will lose your audience.
3. WRITE. Whatever, wherever, just start getting the thoughts out and organize.
Hope that helps.