The #Fediverse is a decentralised world, but some things have established themselves in my timeline over the years. So as a refresher, here a few hashtags you could use, feel free to add more in the replies.

#FediHire when you are looking for a job
#FediGive when you have something to give away or donate
#FediBlock If you want to warn others of accounts/instances to block
#Caturday Needs no explanation ;)
#SelfHost to discuss running your own online stuff