Imagine Reddit as one large city. It contains several neighborhoods that are subreddits. You can move freely between these neighborhoods, but they're all ultimately under the control of the mayor.
Lemmy is meant to look and feel similar, but we have several cities with several different mayors. Don't worry - the mayors have an agreement so that you can visit neighborhoods from all cities freely. But if one city burns down... The others remain up. And if one mayor becomes crazy, the other cities can ignore them.
Apart from that, your experience should be similar. Find communities you like, follow them, post, comment. Just keep in mind it's early days so the apps and website still have a few bugs here and there, they'll improve with time.