Personally, I initially started with following accounts of some companies or softwares that I use or am passionate about like The Fedora Project, The Tor Project, Bitwarden, Blender, Proton, some game engines (and their creators) etc. You can usually find their mastodon handles on their website and/or somewhere on their twitter profiles.

After that I branched out from there gradually, following people as I found them in discussions, the explore page and through some 3rd party discovery tools as well like "Followgraph for Mastodon" which looks up all the people you follow on Mastodon and then the people they follow then it sorts them by the number of mutuals.