I’m a bit in a hurry so I will answer very briefly. You can do everything in blueprints, especially if it’s a rather simple game. I do everything in Blueprints. 
You should check the RTS game in the content examples or tutorials in the launcher (don’t remember where exactly it is). It is about creating a simple tower defense game as far as I remember. Also check the community section in the Forum, there is some free AI and RTS examples.
And otherwise, just go to Youtube and watch some tutorials on how to create your first basic AI, rts controller, damage function, etc. It helps a lot to get an understanding on how the basics of UE4 work, and also help you to start building your game base.
A tip for a beginner, before you start building a game, create a complete concept. You can of course just play around in UE4 to learn it (like making a little shooter game or Tower Defense), but if you want to really build something you need a concept. Otherwise you will waste too much time changing things and working on stuff that you don’t really need in the end.