Stating the obvious, a bestseller has to be a good book. However, there's more to creating a bestseller than just writing. A good book cover is the first element needed to attract potential readers because the cover is first thing a person checks out. There's a great chance that the reader will decide in the first few seconds of scanning a cover whether or not a novel warrants further examination. A poor cover will undoubtedly turn off a reader who isn't familiar with your work, but an attractive cover will garner sales even if the book is poorly written initially. An affordable graphic designer can create your cover online for under $100.
The second element is good marketing techniques because a good book won't sell without the proper exposure. Good marketing comes in many forms, but an author on a budget can use social media or pay-per-click advertising methods in order to boost sales. Google Adwords will even allow you to create a campaign with no money down.
The last element needed to launch a bestseller is getting honest reviews for your book. This can be done by sending your book to book bloggers who review books of your genre on a regular basis, or sponsoring giveaways on sites like Goodreads for example. Amazon Kindle also has a program that allows authors to sell books for free for a limited time period. This will reel in a few honest reviews without spending a great deal of money hiring an advertising agency to promote your work.
Rome wasn't built in a day as the saying goes, so an author shouldn't expect to be an overnight success either. An author can achieve a steady stream of sales eventually by applying these principles and has to be diligent by continuing to put out at least two to three novels per year to build a loyal fanbase.