I don't know about python, but I can tell you how I started with my rts:
Define the map, with support for navigable and unnavigable space.
Define units, and buildings.
Define unit properties and orders (what is a unit doing? standing-by, on patrol, walking, attack-moving), etc.
Sort out movement and unit navigation around maps.
Create UI for unit selecting (not as straight forward as it seems, since you can make multiple selections with a drag, select units or buildings, and select of owned/allied/enemy units)...
... then setup the events for issuing orders with left click/right click functions.
Define units types (and their properties) which serves as a template for creating units.
That should be a good starting point
(ps. you posted in the wrong forum, but it's ok.