Russian Cases (word endings in Russian)

Can any Russians tell me exactly what is intrumental case, and how to use this?

Unfortunatly this f@@in forum doesn't allow to make post of more than 1000 symbols.
I wrote an article in Russian in viki.
Read it
http://www.italki.com/knowledge/wiki-%d0%9e%d0%b1+%d0%be%d0%ba%d0%be%d0%bd%d1%87%d0%b0%d0%bd%d0%b8%d1%8f%d1%85+%d0%b2+%d1%80%d1%83%d1%81%d1%81%d0%ba%d0%be%d0%bc+%d1%8f%d0%b7%d1%8b%d0%ba%d0%b5.htm
or
http://www.italki.com/knowledge/wiki.htm?n=%d0%9e%d0%b1%20%d0%be%d0%ba%d0%be%d0%bd%d1%87%d0%b0%d0%bd%d0%b8%d1%8f%d1%85%20%d0%b2%20%d1%80%d1%83%d1%81%d1%81%d0%ba%d0%be%d0%bc%20%d1%8f%d0%b7%d1%8b%d0%ba%d0%b5

i will tell u 
u must end the word by the right end 
and don't put any worng end in the word 

if u wanna any thing 4m me again just tell me 

the last post is of a real interest)) substantial stuff, don't you know))))))) really pithy) you really have to put the right ending to the russian word))

what are you talking about foof and silver fox? I know that this is important in Russian, you need not tell me. This is fr me to ask questions about it when i have problems with cases.

My opinion is this: you learn russian very well if you'll speak. First time you must only speak without any rules writing. In that case you'll understand all the sounding sistem of language from inside. Doing this you don't pay attention to the mistakes. Believe me there is many people russian who is mistaken with cases. It's not necessary making right the cases for the first time. Only speaking, the rules will be come after that. Good luck for you.

instrumental case  ends  m.-om,em ,n.-om,em,f-oy,ey
with preposition C(russian) shows that i make something with someone(not alone)
я иду С братом-i go with my brother
without preposition shows that somebody makes something with helping something
я пишу ручкой= I write with pen

p.s. instrumental case in plural -all genders end -ami , yami

Can someone explain genitive case for me??