There are actually two different ways to get to your welcome email. You can follow the instructions on this thread, which also gives you some pretty handy tips and suggestions, or you can follow the path below, and still use the tips on the other thread
To enable the Welcome Email, simply go to:
Admin CP -> vBulletin Options -> User Registration Options -> Send Welcome Email -> Yes
You'll notice that you can edit both the title and the body of the email using the links right there.
The Welcome Private Message is just below, and you can edit the title and body on it as well.