Minecraft wiki entry for Defeating_a_pillager_outpost, associated text: Potions such as strength and/or the turtle master (invisibility is ineffective in Java Edition)
Enchanted or normal golden apples
A Trident with Loyalty III, and if wanted Channeling and/or one with Riptide (water must be nearby or you need many water buckets), as another ranged weapon option
A Lava Bucket
TNT with Flint and Steel or Fire Charges to ignite it (these can also be used to burn the outpost down)
Snow blocks and/or iron blocks with carved pumpkins to create golems to help fight (sometimes pillager outposts have targets with carved pumpkins on them, which you can use)
Ender Pearls
Totem of Undying
A tamed Wolf and/or Fox, preferably multiple, to help fight
If a crossbow is being used, bring firework rockets crafted with several firework stars for ammunition and a crossbow
A Milk Bucket to remove the bad omen effect one gets after killing a captain.