Theres a lot of resources out there.
You could:
Use the in-game fireteam finder
use the destiny 2 companion app, which is a fireteam phone app, that is separate from the in-game fireteam finder
https://www.bungie.net/7/en/registration/apps
join a d2 discord, there are many, this one has over 200,000 people on it
https://discord.com/invite/d2lfg
join a clan
https://www.bungie.net/en/Groups/Forum?groupId=2111385or
https://www.reddit.com/r/DestinyClanFinder/
or
https://www.reddit.com/r/Destiny2Clans/
edit: found 2 more sources
Fireteams/comments/1cm7cbq
DestinyTheGame/comments/s6t5e4
find a sherpa (to teach the raid)
https://www.reddit.com/r/DestinySherpa/