You don't so much install Multisite as activate it: first you install WordPress in the normal way and then activate Multisite. But before doing so, you need to decide how your network will be structured—using subdomains or subdirectories.
You can use one of the two structures for URLs for the sites on your network, but you have to choose one before you start:
http://site1.yoursite.com. If you're planning to let people create their own site, you'll need to have wild card subdomains activated for your hosting. Find out more about wild card subdomains at https://codex.wordpress.org/Configuring_Wildcard_Subdomains.