Take food for well fed,
sharpening stone if melee,
sunflower for happy buff,
ammo box if ranged,
crystal ball if mana,
bed in house next to nurse so you can magic mirror and heal with autopause on, good time to calm yourself
heart lanterns for regen
campfire for regen
honey pools for regen if applicable
mana statues and mana potions if magic
heart statues for health (lots!!)
thorns, endurance and inferno potions (every buff helps)
if ranged a homing weapon such as pirahna gun or chlorophyte bullets is good - every hit counts
celestial stone for added buffs
if yoyoer, have yoyo bag
frostspark boots are great no matter what tactic you use
mana flower if magic
have summoned minions such as frost hydra and minion summoning weapons
I've never gotten to moon lord, but these above tips will help with any boss