You will also need SASL and SASL modules (libsasl2 and libsasl2-modules respectively in Debian).
SASL AUTH for outgoing SMTP conections is controlled by ‘smtp_*’ parameters in main.cf.
Quick-and-dirty solution is:
smtp_use_tls = yes
smtp_tls_note_starttls_offer = yes
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_type = cyrus
smtp_sasl_security_options =
/etc/postfix/sasl-passwd:
[smtp.of.your.isp] username:password