Getting a sitemap for your site is simple. You can do it in three steps: generate a sitemap, upload it to your site and notify Google about it. There are two ways to generate a sitemap - either download and install a sitemap generator or use an online sitemap generation tool.
Some of them are free, but they often have a crawl cap on site URLs, so it’s up to you to decide which one to use.
When choosing an XML sitemap generator, pick one that allows reviewing the crawls of URLs and deleting any duplicated URLs, excluded URLs, etc. - you only want to include the pages on the site that you want a search engine to index.
Once you created a sitemap, you need to upload it to your site document root (http://example.com/sitemap.xml) and let Google know about it - this means that you need to add the site to your Google Sitemaps account.
NOTE: In order to be able to add the sitemap to your account, you need to be the legitimate owner of the site.