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.  All the tools within the program regard the schematic as the master copy and the PCB design as the dependent one.

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. In general, only component names can be back annotated from the PCB to the schematic apart from one very narrow exception where the 'Swap Connections' function has been used to ease tracking.