so first thing is if data caps matter you want to limit the number of relays you connect too

a lot of relays are listed here https://nostr.watch/relays/find

you might choose regionally both for language and low ping times

You might choose by uptime, noticing a relay isn’t connecting frequently is a good reason to find a different one

you might want to join a paid relay, at the most basically level this is just a one time fee to gain write access, people sometimes browse only paid relays, because there is a small commitment made that would be a pricy barrier for someone trying to spam with many accounts. Because some only browse global from paid relays, you increase your reach by paying for write access.

there are filter relays like wss://nostr.wine and kind specific relays like wss://purplepag.es/ that only do profiles to make those load quickly

if there are certain accounts you want to make sure you see the notes from, you want to be on the same relays as them.

none of this is quite a settled science just yet.