How do i put my sim card into my old phone?

my phone is broke so i want to use my old one for now, but when i put in the sim card it needs a password, how do i get this password and will using it in the old phone change it forever or can i put it back into my other phone when its fixed?How do i put my sim card into my old phone?
try using the last four digits of your phone number if you never set a passwordHow do i put my sim card into my old phone?
did you set a lock on the old phone?

also what phone is it?

and what type of carrier do you have?