A friend asked me this the other day. He's always been an avid fantasy reader but now he's starting on a novel. He loves it but he's not quite sure that he's doing it "right."
Should he start on page one and write to the end? Should he worry about picking up ideas from fantasies he's read in the past? Should he edit the first chapter until he's sure he has it perfect? And these are all things that new writers think about.
The thing is that when it comes to writing there aren't any hard and fast rules. What works is what you should do, but, of course, an inexperienced writer isn't sure what works. I've always started at the front and kept going until the end, but not everyone does that. If you're getting started, give that a try. But don't be afraid to write scenes out of order if that isn't working for you. Just be aware that if you write a scene in the last chapter, by the time you get there you'll almost certainly have to re-write it. That's ok too as long as it gets you there.
I can't tell you how strongly I advise against editing and re-writing until you've finished your first draft. I have known so many aspiring writers who got stuck writing the same chapter over and over for years. Think about the fact that by the time you've finished, something you decided on later may still mean going back to change what you spent so much time polishing. When you think of something to change, make a note. MS Word has a great function called Comment that allows you to put notes into a Word document. If you write on paper, just put a sticky note on it. But then keep going.
And don't worry if you find some elements of something you've read in your work. In your work, you'll put your own spin on it, make it your own. There are no new ideas, so that will happen. But there are new ways of using the old ideas.
One last thing, read some good books about writing -- ones written by good writers, agents or editors. Most of us start out writing because we love to read, and that's a good thing. But reading really doesn't make someone a good writer. After all, you're busy reading not watching the technique the author used, so there's a lot to learn.
I'll recommend a couple that I consider essential:
"Character & Viewpoint" by Orson Scott Card
"Beginnings, Middles & Ends" by Nancy Kress
"Self-Editing for the Fiction Writer" by Browne & King
"On Writing" by Stephen King