Noob Question about Hashes

I kinda understand hashes i think, but I haven't got a clue how I'm meant to find them.
If I want to gain access to a specific website I used to have an account for, how do I know what hash to use for an attack. It seems like I'm missing something obvious because no guides seem to mention it at all, they all just assume that I have a hash to crack.

Any help is gratefully received.