1) How well the server aligns with your interests and expectations. Some servers cater to people that like technology, software development, certain hobbies, etc. This doesn't impact your overall ability to interact with people on Mastodon. The interconnectedness ensures that you can subscribe to people on other servers and they can subscribe to you so your home page feed does not depend on the server you pick. The server does influence what you see on the local feed though. Each server also has different rules so that is a part of it.
2) The long term outlook of the server. If you join a small server with iffy hosting then you run the risk that the server will not stick around in the long term. That may put you in a situation where you have to migrate your account to another server. That's assuming that a server is shut down according to the Mastodon recommendations which gives the users notice and time to migrate. It's possible that a server could just shut down abruptly and you don't get the notice you need to migrate. You can mitigate this by creating a secondary account on another server and setting it as an alias of your primary account. As long as the alias status is maintained you will be able to initiate migration on the secondary account.
So with that bunch of stuff out of the way. If you are not a technical user that is looking for something specific and you are just interested in getting onto the platform without having to worry about the long term outlook of the server then your best option is probably to sign up at mastodon.social. That is the server that is maintained by the non-profit responsible for the development of Mastodon. It also has the most users last I checked. Sign up here: https://mastodon.social/explore
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