This may sound obvious or maybe stupid ("how can I just write if I don't have a clue?") but from my experience it's best to just throw yourself in at the deep end and swim. I mean, you know that you want to write. You also know that you'll never get it perfect (who does?). And you know that the first hub is likely to be not your best.

Also, you will probably want to develop your own style. For that you can use comments from others and - more importantly - your own analysis. You can only decide what you want to improve, once you have existing material.

You'll need something to write about. You could use the requests for some inspiration, or you could have your own list of subjects that you have knowledge of or strong opinions about. Whatever it is, make sure that you feel confident that you've got enough to create a substantial text. Three sentences evidently is not enough to put into a hub. On the other hand, you probably don't want to write a 100 page novel either.

Personally I use Word to write my hubs. I put everything that comes to mind in a document. Of course all aspects of writing come into play here: choice of words, rephrasing, spell checking, punctuation, logic, etc. Eventually you should have a nice story which is understandable for your audience. Check if there's anything that you left out, anything that is subject to enhancement until you get to the point where you think: "ok, I've said everything that I want to and at this moment can say about this subject".

This is (just like the writing and the topic of choice) a personal thing. Some prefer to write one single text block, others prefer to use headings. As you can see in this hub, I usually (although that depends on the topic) prefer to divide my hub into two or more sections. You can put each section into their own text capsule (as I do), or you can put all sections into one capsule and use the heading text formatting. It's completely up to you.

You may want to add pictures and/or videos. Add the appropriate capsules accordingly and arrange the capsules, so that the appearance of your hub is what you want it to be. I recommend including a comments capsule.

Well then, if all typos have been removed, style and grammar are to your liking, the right pictures and video's are there, you can publish your hub and wait.

With some luck people will not only read but also comment on it. Those comments can help you improve your next hub. Comments can point out where you haven't been completely clear or may have missed things. They're a good source to learn and improve.

You don't want to be a copy cat, but you can learn from others. Have a look at the best hubs and see how they've been composed. See if you can use anything from their appearance, structure and lay out. There's nothing wrong with getting some inspiration.