The usual suspects—
Search engines automatically look for a special file on each site called the sitemaps.xml file. Having this file on your site is a must for making it easy for search engines to discover pages on your site. Sitemaps are essentially a giant map to all of the pages on your site. Fortunately, creating this file and getting it on to your site is a straightforward process.
Most CMS sytems have a sitemaps file automatically generated. This includes systems like WordPress, Magento, Shopify. If this is not the case on your site, you may need to install a plugin or use the free XML Sitemaps Generator tool. The XML Sitemaps Generator will automatically create a sitemaps.xml file for you.
XML Sitemaps Generator
Next ask your web developer or web designer to upload it into the main directory of your site, or do it yourself if you have FTP access. Once uploaded, the file should be publicly accessible with an address like the below example:
Once you have done this, you should submit your sitemap to the Google Webmaster Tools account for your site.
If you do not have a Google Webmaster Tools account, the below article by Google gives simple instructions for web developers or web designers to set this up.