If you're not sure about how to construct a novel, you need to take a novel or two apart.
You do this by going to your keeper shelf and finding several books of the type you want to write. These books should be fairly recent, no more than a year or two old. Pick one by a familiar name writer. Another should be by a fairly new writer with a few books out in that genre.
Here's how you take each novel apart. Start reading the book with a pen and notepad beside you. After each chapter, write down the major plot points and events that happened in that chapter. When you're done, you'll have a good overview of how much goes into that size novel.
Do this for several novels. If you know of a book that is close to how you imagine your book’s plot, be sure to take that book apart chapter by chapter, as well.
Some writing instructors go so far as to say you should analyze the amount of dialogue, narrative, character interiors, etc. To do this, take a number of different colors of highlighters and code each color for a specific aspect of the novel (dialog, interiors, love scenes, etc.) then highlight away.
You need only do this for a few chapters, and you can make copies of the pages if you don't want to mess up the book. You can usually find the first chapter or two on the author's website.
This type of analysis is especially useful for series romances from Harlequin and Silhouette.
I did the chapter analysis of a Dick Francis suspense novel before I started my first suspense, and it was an eye-opening experience about how much goes into a novel.
I know a few writers who have actually used the chapter by chapter analysis of another book to write their own. The result wasn't suitable for selling but few first novels are, and the writer learned a lot about constructing a plot.
If you think your copycat book is sellable, remember that, if you followed the plot and other elements too closely, you may be guilty of plagiarism which is a very bad way to start a writing career.