with the newest xcode version that supports iOS 6.0, you cannot develope for devices with iOS < 4.3 (lowest base SDK), and also the version that supports iOS 4.0 doesn't support iOS 6.0. For older devices you have to set the Architectures to armv6 armv7 in you project-settings tab "Build Settings".
for checking the iOS Version running, just check the available posts here at stack, you will find one that fits for you ;)
All in one I would set the base SDK to 5.1 because it's the last that can be used by iPad 1 and iPod touch 3. gen. and long enought available so everyone should have the latest update.
Older devices can have only iOS 4.2.x and are not supported by current xcode (4.6.2 (4H1003)).