First of all, you should know what your website will be about. This is the topic of your website. When starting to build a website WIX actually asks you this question, to suggest themes suitable towards your idea. You should also have an idea of what content is needed for the website. Is the website built on images, if it is a photography blog? Or/and, will you need an about page for example?
The buttons/arrows which users can use to move from slide to slide can be edited also within the settings. These can be edited to match your websites theme. An animation can be set for the slideshow. This is how the slides come into view. Most animations are a bit crazy and swirl into screen whereas some are more professional and come into view gently and appropriately for a professional website. To edit any text on the slideshow, just double click on any text. Buttons can be direct to other pages of the website or other websites, both using a URL/web address.