Can someone please tell me how to get a drop down to populate as soon as the form is opened
Are you using an external data source for the dropdown values?
If so make sure the data connection queries on load.
That should be it.
We would need more detail on the drop down in question.
When creating your data connection, there is a check box "Automatically retrieve data when form is opened". Make sure this option is checked, then finish the data connection wizard. Test and preview your form, your dropdown should be automatically populated.