Hello,world,miss you all

1-http://en.wikipedia.org/wiki/Kinetic_theory

we study this proof at school and I've never understood it,although a lot of teachers tried to help,but may be I need a little bit special way to introduce this proof into my mind.

my first question:what are the basic concepts I need to understand before understanding this proof?

second:why does the linear momentum of the wall has an opposite sign not the same sign as the linear momentum of the gas particle?

third:what is delta(t)?Is it the time between two collisions?the molecule moves a distance (L) to collide with the wall.then after collision it rebounces and moves another (L) to make a second collision with the opposite wall?that's the way I think of it so I hope I'm on the right track?

fourth:what is the difference between the average speed of the particle and the speed in one direction?

that's the most confusing question to me,

a particle indeed moves with the same speed in x,y and z directions so what is the need to call speed in direction x Vx and speed in direction y Vy

and what does this silly average velocity represent?

I still have much more questions about this subject but that's enough for now.,

Thanks in advance.