In the eyes of anarcho-capitalists, property can only be legitimately gotten in one of three ways.  The first way is through John Locke's "homestead principle," which means that something owned by no one "becomes" your legitimate property when you "mix your labour" with the thing.  In other words, if you come across an unowned field, and you start farming it, then the land you farm becomes your property, along with all of the crops you grow.  If someone else comes along and steals all of the crops you worked on growing, that person has committed a form of force called "theft."  The second and third ways to legitimately get property is through voluntary trade or gift.