Replying to Mac Useres PLEASE HELP!

so i bought a new hard drive (320GB) for my macbook and got it today. so i backup my computer on my external hard drive, with the app SuperDuper!, and then put the new hard drive in my computer. close it all up. and turn on my computer. and so now im on it, but completely running off the external hard drive. the new hard drive doesn't have an operating system so Macintosh HD doesn't exist. compeletly fucked this up. thought it would work better. please help me. a.s.a.p.

thanks