place adsense after the first post In wordpress 3 (twenty ten theme)

Hi people, sorry for my historical question, but I’ve tried everything out there to place ads only after the first post but couldn’t, as in this theme (twenty ten ) provided with wordpress 3, I’m unable to find the codes that are typed in different tutorials, so any help would be so appreciated,
my url is https://onlineforall.info ,I could add a side ads using a plugin, want to add after the first post
thanks in advance