If you are creating a world, create the world. It needs to have it's own structure, magic, how magic works, etc... I'd suggest a quick read from Orson Scott Card, "How to Write Science Fiction and Fantasy". Another suggestion, read Brandon Sanderson, his books are a complete fantasy 101 in terms of worldbuilding.
Then, I'd focus on the Characters. I'd write out at least a page or 2 on each central character to the book. Describe them, in their own words, so you can get yourself in line with how the character will think and act. Essentially an autobiography from that character. If you can do that, you're on your way.
After that, I start to plot a course. Map out where you want to start, and where you want to end. Then fill in the blanks inbetween.
From there, I'd write the opening prologue. Make sure it's gripping and page turning, because if you can't capture most peoples imagination in the first 5-10 pages, you've lost them. Then, I'd write the final chapter and epilogue. Sure this can change, but again it will give you an idea of where you want to go and how you want to get there.
As an example: JK Rowling had written the Battle of Hogwarts in Book 7 before the first Harry Potter book was even published. In other words, she knew where she wanted to go.
As a side note, I'd also recommend having a story that could be continued. Publishers like the idea of a continuing story or multiple books as opposed to a 1 hit wonder type book.
Edit: last points: Write. Write when you don't feel like it, don't just just write about your book, just write. Refine your craft. A good exercise is to find a local newspaper. Find an article, write a story about the article, a work of fiction around the article to attack it in a new way, or a way that would fit your world.
In terms of Worldbuilding. Every world must have a system of magic. Magic usually has laws on how it works and ways in which it works. Otherwise you can just make something up as you go and your readers will know this and you will lose credibility and probably reader loyalty. Every world must have a structure, cities, towns, continents, etc... Granted, it only has to entail the area of the map you are focusing on though, you don't necessarily have to build an entire planet...unless of course your scope is the planet. Next, every map/world, etc.. will have a history. People that rose and took power, wars, famine, disease, etc... Every world will have it's heroes from the past as well.