There are many instances. you’re free to choose one in particular(maybe one specialized in your language or hobby or country) or just go with one of the big ones like mastodon.social. See https://joinmastodon.org/servers
Yes you can subscribe to lemmy communities and new posts will appear in your mastodon feed.
How do I get started with Mastodon? Are there multiple instances to choose from like Lemmy, or do you just sign up on mastodon.social?
Also, it looks like you can access Lemmy communities from there - so how is it different than just using Lemmy?
There are many instances. you’re free to choose one in particular(maybe one specialized in your language or hobby or country) or just go with one of the big ones like mastodon.social. See https://joinmastodon.org/servers
Yes you can subscribe to lemmy communities and new posts will appear in your mastodon feed.
First part: there are different instances.
Second part: I have only heaed but not experienced this cross-talk of Lemmy and Mastodon. So cannot help you there.