You get the superuser app by rooting your Droid. Once your rooted, look in your app tray, you should have a little ninja, that's the way you know your rooted. An easy way to flash new roms is with the Rom manager that's in the market. Now, when you install a new apk, all you do is download it, use a file explorer such as astro (also in the market) look in your download file and choose the apk. If you have not done so already, it will give you the chance to allow apps to be installed from unknown sources, that has to be checked. But make sure to read up on the rooting guides and learn about what your doing. But anyways, I hope I've helped.