I have no SDK

[Catacomber]

I just installed the developer tools onto my macbook and was trying to set up a new program in XCode but my program failed when I went to build it and the message was that there is no SDK of any kind installed. I don't know what Xcode is looking for. I know what an SDK is and I thought I installed a whole number of them. Should I reinstall from the disk? And what is it that I should reinstall? Help! Thanks for any help. I am really a very newbie at Xcode.