Just to add to Archana's information, you want a
Smart Campaign with the following attributes:
Smart List: Fills Out Form
Trigger (not filter), specify the form (you can also click Add Contraint on the trigger and specify the specific landing page)
Flow:
Send Email, specify the email to send to the lead who filled out the form
Schedule: Set Smart Campaign Settings to "Each lead can run through the flow every time" (or whatever setting you think is appropriate. Typically any form fill out autoresponder email is set to run every time)
And then make sure you
Activate the campaign. Once active, the smart campaign will appear as a lit-up light bulb in the left navigation sidebar and any lead who fills out the form should immediately be sent the email.
One final note, make sure the email it is sending is
marked as operational. You can see how to do that here: https://community.marketo.com/MarketoTutorial?id=kA250000000Kz32CAC
This will ensure even leads who are unsubscribed will get the email they requested.