I was an iphone noob in Aug.... bought my first iphone (iphone4) and had it JB in the first 10 mins (jailbreakme.com) since then I have had to use Limera1n and now Greenpois0n and it's not difficult at all! Sheesh... follow the on screen instructions, they walk you through it!
As for Cydia... it's basically just another website / store that you can download tweaks, ringtones, themes, apps that do things Apple didn't think should be standard. If you don't know exactly what tweaks you want, then try Youtube. Search for 'best iphone hacks' or 'best iphone tweaks' and they will show you some pretty neat things you can do!
My Cydia Must Haves:
SBSettings - LOVE this app, and it's free! And it's the first thing I always install after JB for the safe mode
Lock Info - my main reason for JB. I NEED a today screen with all my info and notifications, new mails, sms, weather, news etc all on one screen!
Theming - because i cant stand the default UI look. It's boring and I want to be different from most of the people I know who have iphones

My friends all see my themed up iphone and drool! They are too chicken to JB! My fav theme though is called Glasklart HD.
iBlacklist - because I despise telemarketers! Apple SHOULD have included the ability to block numbers anyway!
3G Unsrestrictor - No more crappy video on 3G vice WiFi, and no more 20 Mb limits on downloads
Live Clock - Just turns your clock icon into a dynamic real working clock
Weather Icon - So your weather icon is not always saying it's 23 degrees and sunny!
I have a few more things installed from Cydia... some ringtones etc. But those are my top for importance. And I'd MISS the functionality of those (and the looks) if I lose the JB.
As for Unclocking... I bought a factory unlocked iphone, so I never have to worry about it.
Anyway if I was brand new at it in the Fall, and learned that much, that quickly... anyone can believe me
