/etc/profile
The login shell executes /etc/profile if this file exists and is readable. This file does the shell setup that applies to all users, so you'll be interested in it if you're the system administrator; you need to log in as root in order to edit it.
$HOME/.profile
The system runs $HOME/.profile whenever you log in, after it runs /etc/profile. If you change your .profile, the changes don't go into effect until you next log in.
ksh's startup file
As described above, the login shell runs certain profiles. In addition, you can have a profile that ksh runs whenever you start a shell—whether or not it's a login shell.