Really stupid question on sign-up forms....

Welcome to the community @KYIO 

Not a stupid question at all!

You need to make sure you have these elements in place:

1. A list to which contacts will be added when they subscribe via your form 
2. That list is selected on the form (see below example). Just select the form's 'submit’ button to see the associated list
3. A flow that triggers when a contact subscribes via your form (see below example)

To edit the emails that are sent when someone subscribes and enters the flow, head to the flow identified in 3) and add emails/make your edits.
 

Let me know if you need any further help

Andy