You don't have any shellcode or anything. You're just calling system() with no arguments. It needs a command to execute. You need to get the address of a string with the command you want on the stack before you call it. I've never seen anyone do it that way though. I've only seen people put the actual machine code to exec a shell in memory and have it return to that. I'm sure the link provided to you will give you a good understanding. Good luck.