Hi, configuring postfix (sendmail) is certainly not easy.
I made a small user-friendly script for a no-questions-asked installation and configuration of postfix with Gmail.
The script configures postfix to relay mail to smtp.gmail.com (which will send your emails to the world).
Check out the https://gist.github.com/3952294
and please tell me if it works for you (or not)
P.S. I checked the script on Ubuntu 12.04, but it should work on previous versions as well. Before running the script, uninstall everything you manually configured on your machine (e.g. exim4 maybe), and also, you should consider purging postfix with
sudo apt-get purge postfix