Stephy, when you start writing there are no rules. The answer is yes to all you questions, as well as no. It's good to start with a story, an idea for a story. The old cliche about writing about what you know is true. If one knows nothing about Botswana, for example, setting a drama there may make one look like a fool. If one writes about a subject that one is familiar with, fewer mistakes are made.
If one is writing a drama - or often any kind of fiction - conflict usually ought to be in the plot. That's what drama is.
Am I being overly simple or painfully obvious here?

As far as a vision for the plot or a character or all the characters is concerned, isn't that going to depend on what you're writing about? If you're writing a whodunit you start with the murder and work backwards, doing your best to hide who done it until you get to the beginning of the story, then go on to the end. The simplest thing is to make yourself the hero, with a little bit of disguise, since of all the people in the world you know yourself the best and no one knows you better than you do.
I have an early day tomorrow. Hope to hear from you.
junco