Hi, I don't visit this forum very much. I am a Technical Games Designer, and I worked at Riot Games for quite a few years before recently founding my own indie studio (Skyborne Games).
however not as Game Designer yet
How am I supposed to become experienced, if noone ever allows me on their team? :C however not as Game Designer yet
You become a Game Designer by first believing that you are one. Don't let anyone, including yourself tell you otherwise. To become a good games designer, you need experience making various types of games. If you're in a Game development programme in University, that's the best place to form small teams to tackle fun game projects. Even if you don't have some of the technical / artistic skills to contribute to a collaborative digital project, you can always start being a games designer by making board / card games.
If you want to be a games designer, there is very little reason not to be making games right now, that's how you beat the catch 22 of becoming a games designer professionally.
For example, I am looking for a book or resource that tells me everything about Game Loops, how to write Game Design Documents correctly and how to describe Game Mechanics.
There are some pretty good talks on GDCVault, I highly recommend the Hearthstone design talk, and the George Fan's PvZ talk.
That aside, it's extremely EXTREMELY important that you balance this out with real experience, the real experience is probably more important than reading the books. (Real experience of playing and analyzing both your own and other people's experiences, AND making game experiences and analyzing other players' perspectives)
The best design books out there teaches you frameworks for understanding / analyzing great games, but that alone doesn't mean you can make great games. Why? Game development evolves rapidly. Game design must evolve with it, to solve new problems that a static book won't be able to keep up.
Also I want to know, what kind of Maths do I need to know as Game Designer?
As a Technical Games / Systems designer, I can tell you that the more math and engineering you know, the more tools you have at your disposal to solve design problems.
Having a solid understanding of probability is a MUST for systems design.
Having a solid base in Calculus 3D / Physics has helped a ton when designing AI and physics based games.
The amount you know in math (& any other discipline really) will limit how you as a Games Design approach and solve various problems.
I hope you have a ton of fun on your journey as a Games Designer!