upgrade 10.4 to 10.5

Hi All

I've been under the impression that my MacBook is servely out of date everything seems to need 10.5 and higher. I am currently using 10.4.11 where can i get an upgrade to 10.5? I don't really understand all thi stuff and any help would be appreciated as I've never been able to put music onto my iphone 4s and the portable hard drive i bought today isn't compatible so i have to take it back

I have a 2 ghz intel core dup, 2gb of memory if that helps

cheers