Hi Susan,
It's in the Setup menu (if you have SysAdmin access you can change it). You can find it under:
Project Preferences -> Tasks & Issues
The setting is called ' Start Date '. You have two options:
' Based on the Project Planned Date '. If you select this, tasks will have an 'As Soon As Possible' constraint. This is what you want I believe.
' Today '. If you select this, tasks will have a 'Start No earlier than' constraint and are set with today's date. We have it set this way because we generally don't want to schedule tasks for the past. If you choose to use the project start date for new tasks and it is in the past, any changes to the task dates need more effort/clicks to change.
Regards,
David