To make a field on your form required/mandatory, follow the steps below:
From the fields page, click on the field name.
Click ‘Add Validation’.
Set the validation type to "Required".
Add an appropriate 'Error Message' such as “Please enter your first name” (this is the message the will display if the field is left blank) and click Save.
The 'Initial Value' is normally only used with Dropdown type fields. For all other field types, leave this blank. It can be set to a value that is being displayed to prompt the user for information. Example: For a Dropdown type field where the first item in the list has Value="Select Car", then set the 'Initial Value' for the validation to "Select Car". The result of doing this is that "Select Car" will be treated as a invalid entry when the validation is performed, so the user will be prompted with the 'Error Message' you have listed.
Test your message by viewing the form and trying to submit a registration leaving the field blank. If you have payment options on your form you will need to select a payment option before the system checks for required fields.