The Fediverse and Mastodon are not super intuitive to get started with so here is an introduction in two parts!
The most difficult thing is to discover content and people you find interesting.
For that there are generally two strategies: first you can post and subscribe to hashtags. This is currently the best way to find particular conversations. For example you could look at or post to #fediverse to find others interested in that topic. Most commonly #introductions is used to describe briefly what you are interested in, that way people can follow you. If you do I can boost you for visibility.
The second strategy is to make use of both the 'Local' and 'Federated'/'Global' timelines. The local one shows all posts by people on post.lurk.org. The 'Global' timeline shows all posts by people followed by people on lurk.org but who are themselves on a different part of the network. See here for a visual explanation: