Although you can in theory start with either PCB or Schematic, if you are designing the whole project then it is better to start from the Schematic. You then use this Schematic to 'drive' the PCB layout contents.
Although there is a Reverse Engineer feature that will create a 'starting point' Schematic from a PCB design, this is generally only intended for use when you have been given a PCB design for which there is no corresponding Schematic, and you need to create the Schematic as 'documentation' for your layout.
Any changes to the 'netlist' (adding, changing or removing components, nets or connections) should always be done in the Schematic. Only Component Names can be back annotated from the PCB to the Schematic.