I am a new user. To me it felt a bit like signing up with an ISP. There’s a list you can choose from, rather than there just being a single reddit.
Once you sign up with an ISP, you can go anywhere on the Internet. Similarly, once you sign up with an instance, you can go anywhere in the fediverse.
Unlike an ISP, there are no costs or fees involved in signing up, though you are more than welcome to help fund the upkeep of an instance through donations. Unlike reddit, lemmy is not a profit-driven operation trying to serve you ads all the time.
In terms of choosing an instance, bigger is not necessarily better. Personally, I went with the medium-sized lemmy.ca as I am Canadian and it seemed to make sense, and have since learned that some of the bigger instances are struggling to keep up with surging demand as people seek alternatives to reddit.
Once you have an account, you can go into Communities, click on the All tab (which lets you see the whole fediverse), and start searching for and subscribing to whatever interests you. Communities are similar to subreddits, and there’s a good chance you will even find one with the same name.
So it’s you…