These are some points I think are useful to have a general idea of:
- What is the main storyline of your novel?
- What are the main events that will occure?
- What events that happen inflict your main story?
- Who are your main characters? What are their backgrounds?
- How do they think, react?
- What are their main character traits?
- What are their character flaws?
- Their bonds and weaknesses?
- How are they involved into the society?
3. The world around them
- What time does your story play in?
- What technology level is present?
- How is your world governed? Is it a monarchy? Democracy?
- Is it in a state of war or are there any local conflicts?
- Are religions playing a role? Which one?
- What kinds of religions/gods/ deities exists (are believed to exist)?
- How popular is magic in your world?
- Is spell casting a day to day activity, that everybody does, ore do only a few gifted wizards and sorceresses exist?
These are just a few questions that could help you get an Idea of how your world could look like. You should note all this Information in some organized way, maybe some local Wiki on your computer?
Whenever you think of new features for your world, look at them in comparison to what already exists. Does it seem fit? Does it contradict something?
Further reading: Consider reading this article. It seems to provide some useful tips about writing your novel.
Have fun writing, and don't forget the Party point on the list!
More on the wiki thing:
I recommended using some sort of local wiki. For the point of which one to use, I recommend having a look at this article on Wikipedia (how convenient :).
Personally, I use DokuWiki for my Dungeons and Dragons sessions, as it is lightweight and simple to set-up (doesn't even need a mysql-database), easy to use and free...
The reasons why I recommend one are majorly simplicity. For example, I like the opportunity to categorize descriptions of different characters or geographical locations by their families or countries etc. You could do this with a Text-File as well, but If you go into detail on your descriptions, that file will shortly become very big and confusing. A wiki enables you for an easy search and versioning of your thoughts. And offers possibilities to easily associate different articles.
You sure can do all of this with a single word document, but from my personal point of view, using a wiki makes it much more organized.