To her 20 million followers, Erika Kullberg, 32, is a cheerful source of personal finance how-tos and hacks (don’t throw out those old Nikes — you could use them to get new ones for free!). But before she amassed her audience, she was a corporate lawyer with $225,000 in student debt. She paid it off in two years and then got obsessed with helping other people gain control of their money.