i think that the first one i learned was something by Run DMC. i was over at a friend's place, he had a guitar & we had Run DMC on. i was maybe what, 9, 10? hehe.
if we're talking about first riffs/songs that we learned when we actually started playing the guitar then mine is probably "knocking on heaven's door". to this day the first thing i do when i pick up a guitar is test if it's in-tune by playing an open G chord.