The start form is a form which must be filled out by a user. With the submission a new process instance will be created.
Declare Startup Form
To declare the start form open the context menu of the form in the process explorer. Then hit the "Set as startup form" option and rebuild your workflow.
Remove Startup Form
The Startup Form setting is stored in the workflow properties of the project. Activate the project in the process explorer and clear the property "Startup Form". Then rebuild your workflow.