I'm currently using C++ for developing an RPG, and I would recommend it over C#. I don't know much about C#, butI have coded in it a few small programs.
C++ is very well known in game development. I would recommend it and a framework such as SFML or SDL. Once you get more familiar with these, you can move onto learning Directx3D and OpenGL.
Learn about Game States (I just learned this the other day and it drastically makes management of game code so much easier), game timing, game loops, Isometric games, sprite-sheets and tiling. The latter two are important for 2D games and since you are building a RTS, It may be essential for you. I know you are not doing a RPG game, but a lot of the material can be used for an RTS.
I'm following a book called Programming Role Playing Games in Direct x, it's not for beginners, but it has valuable information on gaming algorithms for Enemy AI, Game States, Game loops, etc.
I would recommend checking out Coke and Code ( type that into Google ). I think the same person also released a Youtube RPG tutorial series.
3DBuzz has some tutorials, but some are paid. There C++ one also does a RPG game.
SFML Made Easy is another series on Youtube that is really good.
It wont be an easy task, just start small, take your time, and don't do too much at once. Start with just getting a character walking around on a screen ( this will require animation frames, so look that up ).
Understand the update() and render(), calls in a game loop.