A very common type of automation, the Welcome Emails, is based on triggering an automation workflow as soon as someone subscribes to an email list. Designing your own automation, whether it is 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 workflow when someone subscribes to any of your lists.
The only parameter that you need to define on this type of trigger is whether the workflow will be triggered for every subscription made, or whether it will be triggered only on the first subscription per list.
After you are 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 that will be triggered whenever someone subscribes to any of your email lists. Learning more about the available Conditional / Control steps and the available Actions, will help you finalize your design.