A very common type of automation, the Welcome Emails, are based around the premise of triggering an automation as soon as someone subscribes to a mailing list. Designing your own automation, whether it's a Welcome Email or something equally useful, is very easy. First you need to choose the right type of trigger among the available trigger types - the "When someone subscribes to any list" trigger type will fire your automation when someone subscribes to any of your lists.
The only parameter that you need to define on this type of trigger is whether the automation will be triggered for every subscription made, or whether it will be triggered only on the first subscription per list.
After you're done, clicking on the Save button will get your trigger ready and waiting inside your workflow.
This is the first step towards creating an automation which will be triggered whenever someone subscribes to any of your lists! Learning more about the available Conditional / Control steps and the available Actions, will help you finalize your design.