Mastodon: first pick a server. What does that mean? You figure it out. Ok, now select a client and install it. Doesn't matter which one, but it actually does. Then use that client app to log onto the server you made an account on. Now you just need to figure out who to follow and you're ready to participate!
The how-to-search-for-people-to-follow thing caused me trouble with Mastodon. I could handle getting a client and an account, but actually finding people not on the same instance as me was a challenge. Discoverability was pretty broken.
Bluesky doesn't seem to have that problem.
Lemmy I've stuck with because it handles that better.
The latest version of Mastodon does suggest accounts to follow. But you're absolutely right - Bluesky being fully centralized can do a lot of things easier than it is for the Mastodon network. I have high hopes that @dansup@mastodon.social will help alleviate this a lot though with the Fediverse equivalent for "starter packs":
It offers you a server, which is a step up, but the option to pick a different one is still very prominent which is going to make some people ask questions and lead to the usual confusion and anxiety about picking the "right" one.
Anyone who has studied UX and how users move through an app knows that every step you make someone do has a huge drop off in user completion of that process.
Unfortunately that means that centralized, simple platforms will always have a distinct UX advantage over federated platforms. We have to make up for it by being simply better. (No ads is a good start.)
AFAIK, Mastodon has exactly the same experience as BS if you just download the Mastodon official client and pick the main mastodon.social instance.
And as far as finding people to follow, just follow some hashtags to start with. Like #boston should immediately got you local users and news and #mac for computer news, etc etc. Following 5 or 10 hashtags will get you a pretty strong initial feed on mastodon.social, I suspect.