In my opinion by gameplay RTS is the second hardest genre, with MMO being the hardest (harder by several order). So it would probably be better if you forget about the 3D part for a while. If you really want to learn about 3D programming, try make a FPS or TPS instead. RTS has too many stuff to hit your toe with before it can barely become a "game".
Start with a flat, 2D world, model everything as simple circles and rectangles.
Make a tiny circle as your "unit".
Make it move around.
Make a rectangle wall.
Stop the circle from going into that wall.
Make other circles.
Stop circles from going into each other.
Make circles fight.
Make circles die.
Make a map format so it can contain many walls and probably a basic map editor.
Make circle figure out how to get from A to B (pathfinding).
Make circle actually go from A to B (steering).
Make a lot of circle go from A to B together without traffic jam (boids or other techniques).
Make simple AI for circle auto attack, etc.
Steps above had took me 4 week solo for a prototype our studio was investigating, and another 2 week for LAN multiplayer. Definitely doable but some of the subjects are hard, especially for beginners.
And you have a full model for your RTS world, then you can try to render it in 3D. Make your tank go up and down, pretending it was following a ramp while it was actually in a 2D world.