How do i customize what applications i wish to have running?

u know in task manager when you go to applications and it shows you what application is using however much memory, how do you change this to just have the important ones running and to not allow others like 'google toolbar updater' and 'apple mobile device driver' to start up...if you know wot i mean. and which ones are vital to keep running and which are just little irrelevant annoyances slowing up my pc