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?
Similarly to adding almost anything to your site, to add a gallery, you use the add element again. Each gallery you will see will be different from each other. From professional looking galleries to the less so professional. Wherever you choose to place a gallery on your site, always make sure the images are appropriate for the page or your website.