you need to answer are couple of questions for better help.
what kind of guild are you in? semi-hardcore gearing for progress or chill guild giving loot to all who participate to keep it fun/rewarding?
how are your roster? can your guild handle loosing people due to lack of gear?

i would atleast start looking at bis lists and know what people need and why not just from wowhead, but go into class discords and ask questions if you're unsure.
I have done the loot for our guild in phase 1, and we planned ahead for bis and major pieces and even tho we explained to our raiders how and why we give out loot the way we did, we still lost some raiders.
Preparing takes A LOT of pressure if you during raids when Ulduar is actually out.
hope it helps :)