Here are a couple of articles about getting started--getting ideas and getting good characters. I know it's a little late to post this (for those of you doing NaNoWriMo), but the articles are good and I think they're worth a look. If you are writing a novel in November, consider this a checklist.
First, You Need a Good Idea. Alexandra Sokoloff gives a great checklist for what a good story idea is. One thing writers need to realize is this: even if you have the BEST idea for a book, if it's not a story that gets you pumped, if it's not a story that gives you chills or makes you giddy, it's not a good book for you to write.
I could never have written Dan Brown's books, even if I had had the ideas first. I just wouldn't have done as good a job.
I think this is why we as writers probably shouldn't worry too much about people stealing our story ideas. There are exceptions, of course. Don't be careless ;)
Second: Show Some Character! This article focuses more on characters of YA fantasy, but I think the advice can be applied across the board. Make sure that your characters work hard for their happy endings. It's important that your readers connect with your main character. This is a great article that analyzes a common problem that doesn't get addressed very often.