Novelizing fairy tales is fun, good, and easier when beginning with writing.
I do not mean to say it is easy, for it is not, but having a base to work on can be helpful, especially if you are new to writing.
First, find a fairy tale that you like.
It needs to be one you can work with, so chose carefully. If it is your favorite fairy tale, that is great, because you know it well, and you like the characters and setting.
But it has to be one that have some un-answered questions, ones that you have to answer.
There is a general rule when re-writing fairy tales.
If you are trying to just re-write it, then:
Try to explain, defiantly add, but change try as little.
If you are trying to make it recognizable, change a few things, but never completely change every thing.
If you are changing big things, explain why in the fairy tale they were like that, for example, say the evil queen actually liked Snow White, tel us why she wanted to see her all the time, maybe she thought Snow White was really beautiful, so she watched her in her mirror so that she could get some beauty tips!
If you are changing it completely, good for you! As the writer, you can do absolutely anything, and no one can tell you not to! At least not now, But right now write what YOU want, and worry about publishers later!