Secured credit cards

My friend is 24 and a young mother. She has never had a credit card and she wants to build her credit. Her income is somewhat limited at the moment because she has a new baby and only works part-time, but she wants to be able to pay for small daily expenses with a card that she can pay off in full every month. She is not seeking a large credit line. Even $100 would be sufficient. Can you provide some advice on what kind of card and where to start for someone looking to build good credit? Should she try prepaid credit cards? Any advice would be most sincerely appreciated. Esmeralda, Marlborough, MA