First of all, you need to study the characteristics of your site (or at least the most complex ideas about it): whether it uses a CMS and what kind of CMS, whether it needs support for web programming language and what kind, what daily traffic is planned, what content will be located on the pages (how much RAM and disk space is required to run one page, for all pages), whether additional space is needed for caching, backups, etc. Ideally, you should find out the average size of the pages generated by the engine in the server's RAM at https://hitme.pl/hosting-www/. Then, multiplying this data by the number of simultaneous connections will get the average amount of memory consumed.