Gaspard, I mean no offence, but Unreal engine 4 is a low level game engine, so you’ve really got to start from scratch. There really aren’t any specific way you HAVE to do these things, I suggest you just play around with some basic stuff first. Maybe try randomly spawning lighting every 5 seconds (very easy BTW). If you are knew to game development, or have no programming experience, it can be easy for some simple stuff, but you’ll need to develop a greater sense of logic as you move on, because a lot of things in game development do not necessarily require knowledge about the language or engine, but simply a strong knowledge of “how things work”.
UE4 is very powerful, I recently started using it in March 2015, and am already working on a serious game similar to Terraria, I have been using blueprints only. I can not think of anything that you need C++ only for, but if you’d like to make a game the size of Dying Light, you’d need C++. Something like a CoD game could work fine in blueprints, but due to a slightly lower FPS, you’d want C++ for a larger game.
Here is a really fun & great tutorial that teaches dynamic spawning, I recommend you watch it, it’s just 25 minutes long and is quite fun, it is put together by Epic so it’s quite nice. It should really help you with your Slenderman character, it just shows how to spawn things in random locations, and in a custom range: https://www.youtube.com/watch?t=395&v=9-7SmkasS_A