If all you want is root mail to be sent to another account. Assuming you do not already have a dot forward file in roots home.
#cd ~root && touch .forward
#echo email@example.com >> .forward
#echo "test forward" | mail root@localhost
An email should now be in the account specified above.
If you want to have all mail forwarded to a different mail gateway then you need to uncomment and change the following line within /etc/mail/sendmail.mc (assuming RH or variant)
define(`SMART_HOST', `add your email gateway here')dnl
#make -C /etc/mail/
#service sendmail restart
#tail -f /var/log/maillog
Should get you started.