Click on a username or avatar and you'll go to their profile page. There you'll find a follow button.
Why can't I see a follow button anywhere?
What device are you on?
If you're using a laptop/desktop computer, then you should see the follow button below a user's profile picture as others have mentioned.
If you're on a mobile browser, then a user's profile shows their questions and answers at the top of their page. If you scroll down though, you'll eventually get to a profile pic, some stats, and a follow button:
When you click on someone's avatar or name, it will take you to their profile. Right under their picture, you will see " Follow ". Right next to the persons name.
Impossible Nothing