How do I make sub-lists?

My main list is numbered: 1, 2, 3, etc. I want to add a sublist under step 2: a, b, c, etc. I have a feeling that I will have to create a new paragraph style, and I tried to do that, but I'm not sure how to do make letters instead of numbers in the paragraph designer window or how to start at "a" instead of "4".