I would say listen to some and pick one you like -- don't pick it by reading: listen until you hear a ballad that haunts you, then read Child's notes about it, ask other people for versions of it (ask here), see where it leads you. Maybe there is one you heard in childhood -- e.g. "The Cherry Tree Carol" or "Barbara Allen." Or maybe you have a favorite singer (Dick Gaughan, Kate Rusby, June Tabor): investigate which ballads they sing until one grabs you. It should be fun.