Say you have a private key in PEM format, and you want to use that key for SSH into another server, by adding an entry to your ~/.ssh/authorized_keys file with the public key of such PEM file. The following command will parse your PEM file and output the required RSA format used in authorized_keys:

ssh-keygen -y -f path/to/file.pem

This will output a ssh-rsa AAAA… string that is safe to append to your ~/.ssh/authorized_keys. The ssh-keygen uses the -f flag to specify the input file name, and the -y flag to read a private file and output the OpenSSH public key to standard output.