If you want someone to follow you on Mastodon or the rest of the Fediverse, you should give them your account's Fediverse address.
You can find your address on your account's profile page.
Fediverse addresses come in two parts:
@ (Your username) @ (Your server)
For example, the account I'm posting from is @ FediTips @ mstdn.social (but without the spaces).
By combining a username with the server's name like this, you get a unique address that no other account has.
You can follow an address by pasting it into the search box when you're logged into Mastodon etc. This will bring up the account's profile in search results, and you can follow it by clicking "follow".
The reason it's longer than addresses on centralised services like Twitter etc is centralised services run entirely on one site, so they don't need the second part of the address.
The reason it looks like an email address is because the Fediverse and Email are both decentralised and use a similar server-based model.
@feditips You could also give them a URL, right? If I paste "https://mstdn.social/@feditips" in the search box, I likewise end up on your profile page.
@robryk
Yup, you can usually do that too 👍
Have to be careful though as URL method doesn't work with all Fedi server types, for various reasons.
For example Owncast forwards any attempts to look at an Owncast profile to the profile's streaming URL, and putting the streaming URL in the search box on Mastodon etc does nothing.