There are several places in the application where you can add a new individual into the database. In each case the main edit menu for individuals is invoked with new unique key values.
To add a new individual who is not related to any existing individual in the family tree you go to the top level services page and click on "Add Unrelated Individual". If there is no likelihood of the new individual being connected to anyone in the existing tree, you should consider creating a brand new tree.
If the new individual is a child of an existing family then the individual should be added as a child of the existing family.
If the new individual is a spouse of an existing individual then you should add a marriage to the individual and then identify the new spouse.
If the new individual is a parent of an existing individual then you should add a new set of parents to the existing individual and then define the husband and/or wife of that set of parents.