For all things ‘fediverse’, Mastodon in particular, try https://fedi.tips/ In particular you are likely looking for how-do-i-follow-someone-on-mastodon-how-do-i-unfollow-them and the “using account addresses” link there.
tl;dr: assuming you are using the standard web interface, not a client app:
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in the Search or Paste URL area type or paste the complete account address, e.g., @TiddlyWiki@fosstodon.org – If you just search for a partial address you may or may not find them, depending (I think) on whether the server you use ‘knows’ about them yet.
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In the results, assuming there is only one, either click on the + sign or click on the result itself to access their profile and follow them there.
One extra tip: on the profile page of the person you are following is a Notes field. This is for you to use - no one else will see this. You can leave a note for yourself about why you followed that person. Something I learned Fedi.Tips: "The profile notes feature on Mastodon is very use…" - Mastodon 🐘 - @feditips@mstdn.social is a good account for newbies to follow.