tl;dr: assuming you are using the standard web interface, not a client app:
in the Search or Paste URL area type or paste the complete account address, e.g., @TiddlyWiki@fosstodon.org ā If you just search for a partial address you may or may not find them, depending (I think) on whether the server you use āknowsā about them yet.
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 useā¦" - Mastodon š - @feditips@mstdn.social is a good account for newbies to follow.