One of the single most important aspects in writing that often gets overlooked by first time authors is word count. It is true that a story should come to a natural end whenever that might be, but word count can not be overlooked entirely and can play a crucial role for first time authors. The best story ever wont even get a look if it doesn't fall within certain guidelines that vary per genre. Famous authors can get around these guidelines because their name alone will sell books reducing the risk involved in printing large 1000 page plus novels but this is the exception not the rule and never assume to be the exception. For the average adult novel a word count between 80 to 99 thousand words is acceptable but be careful with novels that break 100K words because of the mental aspect. A novel with 99,999 words seems more attainable, similar to the logic marketers price things at $9.99 instead of rounding. Science Fiction novels are an exception because they take up a lot of words just to explain the rules of the world they are set. A first time science fiction or fantasy book could be between 100-120K words but for first time authors aiming for lower is better because the less pages and ink used to print is less the risk for publishers. Shorter novels might get overlooked as well depending on the publishers requirements but tend to do better than excessively long manuscripts, and can be published as a novella.
As a general rule before beginning the writing process do some research into the genre and come up with a vague guideline. Its much easier to figure out the end goal before getting too far into a novel and realizing the word count is going to be a problem. Summarizes a long novel can be a hard process, and adding additional text to pad a low word count is even riskier. Adding unnecessary scenes and dialogue could take away from the story. Never force a novel out of something better suited to a novella or a short story because the readers will pick up on the fluff material and it will hurt the story more than anything.
Here's a rough idea for what the word count of different genres should look like, although depending on the publisher and personal opinion it can vary a little. Always heavily research the requirements of different publications before submitting content.
Short Stories: Less than 10,000 words
Novellas: 25,000-50,000 words (These make great e-books, and may be published in magazines devoted to fiction works. Some classic novels are actually novellas including Animal Farm)
Preteen Novels: 40,000-60,000
Young Adult: 50,000-75,000
Science Fiction/Fantasy: 100,000-120,000
A common mistake for new authors is to put more emphasis on page count than word count. Although it's good to see the pages increasing, changing the format will change the page count however the words will stay consistent. Write what feels right and don't force the novel into a certain length but don't ignore it entirely.