Read as much chick lit as you can get your hands on. Take notes on what you like and dislike about each story and its characters. The more research you do on the types of chick lit novels that are selling, the better prepared you'll be to write your novel.
Create your main character. It is ok to model her after your own personality, but make her funnier, wittier, smarter or more interesting than yourself. Give her flaws as well to make her more believable to your reader.
Make character sketches for your secondary characters that include as much detail as possible, from relationship details to physical descriptions down to their favorite brand of underwear. This will give your characters more depth as you begin writing.
Decide on the point of view that you will write your novel in. You can choose between the first person, where the main character narrates from her point of view; multiple first person, where a couple of characters narrate from different points of view or third person where you narrate the story knowing all the details of what the characters are doing and thinking. Narrating in first person is a popular choice amongst chick lit writers because it allows the reader to feel closer to the main character.
Come up with your story and plot details. Include at least two major conflicts that your main character will face throughout the novel, one towards the beginning and another near the end. Pick many small roadblocks that she and the secondary characters will go through as well. This keeps your novel from being static, and it gives your characters the chance to grow and develop throughout the story.
Write a detailed outline of your plot so that you have a solid point of reference to go off of once you begin writing.
Sit down and write. Make a realistic writing schedule and stick to it. Finding the time to write your first chick lit novel may be the biggest challenge, but soon you'll be cranking out those pages and looking forward to writing your next juicy chapter.