MEMORY

hi! I am still confused about the system memory such as the speed, how to use it and how to buy it so that I can put in my system, my understand is that the memory must match with the cpu speed? sometimes I put the memory in my sytem, I got blank screen, did I get the wrong kind of memory, does the memory must match with the system bus speed as well? when buying memory I read the lable but still confused, please help, thanks