To automatically create a new task from a Submitted Form, you will either edit an existing Form Template or create a new Form Template.
To add this action to an existing form select Forms > View Forms, select Edit on the Form you would like to add the Action to > Edit Form > Actions > +New Actions, Select Create CRM Task from the Action Type Drop down Box and fill in the details for the specific Task that you would like to be completed once the Form has been Submitted > Save.