I highly recommend reading http://www.postfix.org/BASIC_CONFIGURATION_README.html
It's easy to understand, and if you follow it in sequence your postfix should be working just fine.
For a basic setup you shouldn't have to touch master.cf , only main.cf - and only the parameters listed in the link I mentioned above.
Briefly, you'll want to set the following parameters in main.cf:
myhostname = host.name.tld
mydomain = name.tld
myorigin = yourdomain.com
Then do a
if postfix is already running.