The newest pi is a raspberry pi 2 so if the pi you currently own is not the latest rpi it is not a raspberry pi 2. It is likely a raspberry pi B+. Easiest way to check is to count the raspberries when the pi boots. If there is only one raspberry you do not have a raspberry pi 2 and therefore you need to install the raspberry pi 1 sd image.
Transferring roms is also explained clearly on the aforementioned wiki page. If there is any confusion see the linked video on that wiki page.
You don’t have to delete everything rather you need to create a folder called retropie first on the usb and then plugging it into the pi will generate the needed folders. Then you’ll transfer your roms into their respective folders