get graphics up and running so you can render things(Some graphic API). also meshes shaders all that
set up sound so you can play sound and stream sounds(your fab sound API).
design UI system dialogboxes and things.
work out what terrain type you want to use .
create terrain get working renders culls you will need to make tools Eg map editor.
find data structure to hold game objects for fast removal and incerting based on your game type.
make particle systems get this working. fire, water, bombs.
design how your objects will use graphics.
work out a rendering system .
processing system(where all objects get updated).
work out motion component.
work out what type of weapons are needed get them working make them a component and not hard coded(fire type projectiles or straight motion)
create game objects.
just some basic steps I follow when working on my RTS games just to get them up and running the you refine things.