There is absolutely no sure fire way to prepare for parenting. You could babysit for years, get a degree in childhood education, take on aunt or uncle duties, and devour all of the books available, but nothing can compete with getting your hands dirty raising your own kids.
Still, despite the fact that parenting is the type of experience nothing can prepare you for, there are plenty of people out in the world ready to dole out well-meaning advice about how you can know if you're "ready" to have kids.