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?
On the slideshow you will get the option to manage the slides. To edit the images o your slide show, click through the slides and choose slide background. You can choose from your own library of images, WIX’s own and even set it as a colour. Different slides will also have different features within them such as boxes and buttons. In the background settings of the images, always use scale to fit unless you want it otherwise. You can also choose for the slideshow to auto play, how long a slide shows for and other options.