Been a while since I set this up, but these directions should get you going, or at least pointed in the right direction:
1. Go to Admin panel -> Settings -> Autoresponders (Global Setting)
enable New Ticket.
2. Go to Admin panel -> Emails -> Templates -> osTicket Default Template -> New Ticket Autoresponse
Edit this portion of the template to your liking.
Click Save Changes are the bottom.
3. Go to Admin panel -> Departments -> click on your departments and make sure that under Autoresponders that New Ticket is enabled.