Setting when manually creating a new user
People > Create User
You have a checkbox "Send Welcome Email"
Setting when you allow users to create their own account
People > Settings > Registration Settings
Under "New Account Settings" you can enable/disable the welcome email
Content of the emails can be changed if you go to
System > Settings > Message Templates
Look for "New User Welcome Email (Email sent to welcome a new user after they register.)"
Or "User Creation Welcome Email (Email sent to welcome a new user after their account is created by an admin.)"