What do I do so that only specified components are selectable, and the rest are non-selectable?
Use the "Locked" feature (check the "Locked" box in the component properties) for the components you don't want to be able to select.
Or, if you want most of your components to be locked and only unlock certain ones, you can select all components on the board, open the PCB Inspector, make sure that you only have Components displayed by clicking the text next to the "Include" text at the top of the inspector, and check the "Locked" box. This will lock all of the components. Then you can use the PCB List to unlock specific components.