I currently have a level 21 Undead Subtlety Rogue and I honestly have no clue how to organize the action bars. How do you know what attacks go where? etc etc. Thanks!

Hey there I'm an 85 rogue. First off, I would recommend getting the action bar addons Bartender 4 and Button Facade for easy arrangement of action bars and changing of skins. Honestly where your bars are doesn't matter very much as long as you know your keybinds well. Organizing keybinds is more important than organizing action bars. Do NOT be a clicker. Please. We don't need another one of them....
But yeah keep your main abilities close at hand like #1-7 and then any cooldowns as those with shift/alt modifiers (if you don't have like a razer naga). I have my kick on F for easy access and some other CC on R and G and a few other keys. It really depends on what is comfortable for YOUR hand.
As for other parts of your UI, I would recommend IceHUD and a cooldown bar like coolline. They make keeping track of health and cooldowns and energy very easy. Timer addons are also good to have, especailly for like assassination rogues and their envenom buff. (SnD does not really need a timer, as it literally never falls off).