Optimize game for beginner understanding

Hi. As first I want to say that I’m using Unity 5.3. I started in this program about week or two ago so I’m not such as expert and I was looking for how to optimize game but it’s kinda hard for me, as begginer and total noob in this stuff, to understand it and make it good. I’m getting big frame drops like for 5 - 15 fps, even when I’m looking there where is the most stuff from so far. There are water4advanced (4x), several dozen trees and about 70/80 particle (they are soo far from this place when I see on this all and frame drops are for 5 - 15 fps so as I think, at least I, that I think they are not so big problem with frame drops as they are so many but I guess they are too far to make these troubles etc.), and others of course but others are normal objects which shouldn’t be much as problems with it. As next one is that I’m using for this game FPS controller from import package from Unity.

I do really please in helping me in optimize it as it’s impossible to play in 5 - 15 fps to get fun. As second I need as easy explaination as it’s possible 'cuase as I said I’m begginer and I would not understand something, but let’s try.

The processes in your game run in a pipeline: User input is captured, processed, physics and other internal processes are calculated, and then graphics are pushed to the GPU for rendering on the screen.
The performance of your game is limited by the slowest point in this pipeline - the bottleneck that is holding up the rate at which data can be pushed through.

Therefore the first stage in optimisation is always identifying what part of your game is causing the bottleneck. We don’t know that, but you do - using the profiler: https://unity3d.com/learn/tutorials/modules/intermediate/editor/intro-to-profiler

Find out what’s taking the largest amount of time per frame, and optimise that.