my macbook software please help!!!

Hi i have a 2009 aluminum unibody macbook model A1278 and i recently bought 10.6.3 i thought i was going to erase my hard drive and upgrade my computer but that wasnt the case i erased the hard drive but it says that it can not be installed and i have also lost my original install discs moving back to college so i wanted to know if i have to get the original retail dvd of osx 10.5 or can i start fresh and get 10.6