Help! I am giving away my iPhone 3Gs to my sister along with it's data plan, I want to delete my iCloud account on this phone, will delete my iCloud account too? And what about deleting my personal datas on iPhone, will it delete my data on iTunes?

All you have to do is this: Sync your 3GS to create a current backup. Then either: Settings>General>Reset>Erase All Content & Settings or restore it as a "New" device in iTunes. Both accomplish the same thing, erasing all of your data & settings from the phone. You can now give it to your sister, she can plug it into her computer, name it what she wants & sync her content to it. You then restore your 4S from the backup of the 3GS you created when you started. Follow this by syncing your content to your new phone & your 4S will look just like your 3GS.