You can find someone's Mastodon or Fediverse address on their profile page. (See the attached photo for an example!)
If someone asks how they can follow you on Mastodon/Fediverse, then give them your Fediverse address.
They can paste the address into the search box, and then your account will appear in the search results. They can then follow you by clicking on that profile.
Using a Fediverse address is just like @-ing someone on Twitter etc, but with a bit extra on the end so that the Fediverse system can tell which instance they are signed up on. This allows people on lots of different instances to communicate with each other.
(Email has similar-looking addresses for exactly the same reason, because there are lots of email providers out there, and it needs to know which one you signed up on.)