Choose the system you feel most comfortable working with. Then, please read articles on MonsterPost. Here we have many useful posts related to CMS choice. These are, for example, Build a Website on WordPress, Joomla, OpenCart in 5 Days, or Magento vs. Prestashop. Here is a detailed overview of the pros and cons of different CMSs. So, it will be easier for you to choose what will be more convenient. Then, based on the selected system, use the left sidebar and specify the theme's criteria. Among them is the type of template, tags, features, and version of the selected CMS.