Designing your own automation workflow is easy. The first thing you need to do is choose the right type of trigger among the available trigger types. The "When someone subscribes to a specific list" trigger type will set your automation in motion when someone subscribes to a particular email list.
Let's see how you can do that:
1. Scroll down until you find the"When someone subscribes to a specific list" trigger and select it by clicking on the arrow next to it.
All that this trigger requires is that you define the name of the list to which your subscribers need to subscribe, in order for your automation to get triggered. You can find the available email lists on the drop-down menu; keep in mind that only subscriptions to this specific list will trigger the workflow - the rest will be ignored.
2. Select the email list to which your subscribers need to subscribe for the automation to get triggered. 
After you're done, click on the Save button to have your trigger ready and waiting inside your workflow.
This is the first step in creating an automation workflow that will be triggered whenever someone subscribes to a specific list. Learning how the available Conditional / Control steps and the available Actions work, will help you finalize your design.