Welcome to the big family :-)
You are making a right choice of picking SGSII as your first android smartphone as it is one of the most easiest phone to handle, use, update or upgrade.

For rooting and flashing custom roms, i'm afraid you have to read and know what you're doing. And it's also good to know why you doing it.
There's nothing wrong with rooting/flashing but i advice you familiarise with your phone first. Try to know each of every apps that comes with your phone. That way you'll know what app doing what, either that app is important for your daily use or not. Later on you can freeze (by rooting ) or uninstall any unwanted apps from your phone without distrupt your daily usage or the phone system itself :-)

Few apps that good to have for stock rom
1) Android assistant
2) 3G watchdog (if you're on limited data plan)
3) app2sd
BTW for rooting you can take a look at xda-dev Chainfire thread
forum.xda-developers.com/showthread.php?t=1103399