STEP 1: Design
There are two main areas that need to be covered at the design stage of a website build and they are UX and UI.
UX stands for User Experience and relates to the way a user is able to navigate a website. For example UX is concerned with how many clicks it takes to get to a product page, how many pages in the checkout process. So UX relates to how easy a website is to use for a customer.
UI on the other hand stands for User Interfaceuser interface and covers the look and feel of a website. So colour schemes, imagery, fonts, whitepace, being responsive and able to resize correctly depending on the the type of device being used, all fall under UI. (This last feature is another seo 'must have' and is known as being mobile friendly - which is a bit of a misnomer as most web designers now bUIld specifically for mobile and then simply adapt the design for tablet & desktop.)
Getting either UX or UI wrong means customers will not enjoy the experience or get frustrated because they can’t find their way around. If either of these reactions occur they will simply leave.... and probably never come back.
How many people leave your website, known as bounce rate, is one of the metrics that search engines use to determine a sites ranking score. Basically customers like easy to navigate good looking sites and so do search engines.
Note: if you use an off the shelf solution to build your website like a Wordpress blog or Magenta ecommerce solution then the UX is usually OK out of the box since these solutions have evolved with the changing requirements and got better and better over time. Similarly there are a host of predesigned templates readily available for most popular website solutions - which takes care of the UI as well.