Quantum mechanics in macroscopic systems I don't understand the superposition principle in quantum mechanics or the collapse of wave-function (I think it's impossible for me to understand it) My question is: 
Is it possible to demonstrate the quantum mechanical behaviour (Superposition and wavefunction collapse, etc.) in some macroscopic systems under certain conditions so it may be better to understand it? Is it possible to make the wavefunctions of atoms in some material more or less coherent?
 A: You've asked two questions. Firstly is it possible to see superposition in macroscopic systems? The answer is yes. This article describes making a tiny "tuning fork" that can be put into a superposition of different vibrational states.
Secondly you ask "Is it possible to make the wavefunctions of atoms in some material more or less coherent?". Again the answer is yes, and a good example is a Bose-Einstein condensate.
As Gugg points out, there is a good Wikipedia article on Macroscopic Quantum phenomena.
A: There is a Wiki page on this. I take it that you will never "see" typical quantum behaviour, but quantum mechanics is the only way to explain what you do see (especially in typical QM experiments).
A: The basic concept of the collapsing of the wavefunction can be understood be the following Double slit experiment. However my answer does not cover your aspect for a macroscopic question. First focus on basic concepts and than expand your knowledge to the not-so-easy to grasp macroscopic effects. Then consider the beautiful effects described by the John Rennie answer.
Consider reading Double slit experiment which results in a visible pattern. Incident photons/particles have to be considered as a plane wave $\Psi(z,t)$ in z direction. Propability distribution $|\Psi(x,y)|$ of the wave-funkction is proportional to intensity on observation screen.
This experiment performed by Young astonishly can be performed with electrons and even larger particles like neutrons. The conclusion is particles that have to be modelled as waves (under certain conditions). The solid picture of particles still helps in other models.
Interference with oneself
This is easly more confusing. For a start neglect this fact. For advanced understanding of this concept, try to depict the equal experimental result, if one particle at-a-time is used in this experiment. (Somebody can edit an example of this in my answer)
