How do i send a letter?

ok so yes i am stupid, i am australian but ive never sent a letter and i need to figure out how, so i hope someone can help.

what i want to know:

what information i need (address, name?)

how do i format it? (a photo example would be nice)

how do i send one?

my bad if i get confusing, i dont know what im doing

also i am sending it internationally (sending it to poland) if that changes anything