To begin with, you should focus on where there is actually room for improvement (if you don't already know this). In addition to the main goal of increasing sales, a conversion does not only refer to purchases, but may also refer to the following goals:
Add item to cart
Registration for a trial month or product sample
Registration for free version
registration for the afghanistan telegram screening newsletter
competition participation
information request via contact form
Download eBooks, white papers etc,
...
No matter which conversion you focus on, the question is always at which point the interested parties drop out. Using the example of an online shop that wants to generate more sales, it could now measure at what point in the purchase process most users leave the site. Are users perhaps overwhelmed by a payment process that is too complicated or do they miss additional payment options such as PayPal? Perhaps they don't want to create an account and are looking in vain for the missing "Order as a guest" option? Or are the interested parties dropping out before they have even put anything in their shopping cart? While many of these questions are difficult to answer with numbers alone and initially remain hypotheses, after the first step you will know where there is a need for improvement. The concrete optimization then takes place in the next steps.