You can follow someone on Mastodon by going to their profile page and clicking the Follow button.
You can unfollow them by clicking the same button again. The button may have a different label after you’ve followed someone (Unfollow, Following etc), but it will be in the same place on the screen.
If you’re trying to get someone’s profile page to appear so you can follow them, see the sections on discovering accounts to follow and using account addresses.