I've been looking through various supplies to buy for my campaign as a Bloodrager/Fighter and I found that coffins are a thing. So I'm wondering how much damage would it do? would it be like a greataxe/greatsword or a club?
It's an improvised weapon
The pathfinder 1e Core handbook has all the details in the weapons section, you mainly get a -4 penalty when using it.
Catch offguard and improvised weapon mastery feats can help you minimize the penalities for using it and even help you get a little edge with them.
Deciding how much damage does takes a bit of a subjective approach, as the manual says you should apply the damage of a similar weapon of the same size.
Taking in account that a coffin is a blunt weapon that's bigger than the person that's suposed to be inside it, I'd, personally compare it to a large greatclub, this would make it do 2d8 blunt damage, and would add an additional penalty of -2 for the size difference with the wielder thus giving you a -6 total penalty should a medium creature use a medium sized coffin as a weapon.
As using a two handed weapon that's a size larger than the character is not possible, you'd need to either get 2 levels in the Titan Mauler barbarian archetype, or look for a way to consider the character a large creature (like using the enlarge person spell).