If your submit button operates in the standard manner (i.e. submitting saves the form to the library or list) then you would simply need to change the start up behavior of the workflow.
You can change the workflow startup options from workflow settings page.
OR you can do:
Simply add a field to your form called my:Status and set its value (via rules) when the user clicks Save or Submit. Promote this field, i.e. make it a promoted property.
Then, in the SharePoint form library, you can use the value of Status to determine whether the workflow starts or not. If Status = Submitted, start the workflow. Otherwise, don't start the workflow.