The requirements depend on each country. For example, in the case of the USA, the woman who wants to be part of this process must undergo medical and psychological check-ups.
Normally, they are women who have already formed a family, who have their own children and want to help other people. In addition, they must have an interview with the intended parents to get to know them.
This is because, in the USA, the pregnant woman chooses the couple whose child she wants to gestate and the future parents or intentional parents choose the surrogate. We must keep in mind that this is an agreement between two parties.