To simplify there are 3 ways you can create a XML sitemap.
Google Webmaster Tools has a good article of how to create a XML sitemap that you really should read.
There are many online tools out there on the internet. We have picked up a few in a list belove.
If you have a developer / developer team you should tell them to create a sitemap.xml and register it in Google Webmaster Tools. It will help Google to find all your pages and will also give you a better change to get higher rank.