Sounds like he made a couple of rules mistakes himself. First of all, in Dawn of War, the player who deploys second cannot deploy any models within 18" of a model deployed by player one. Second I'm not sure if I'm reading this right or not but it sounds like he used his Waaagh! on turn one which is also against the rules.
I think your biggest mistake may have been putting your destroyer lord in with a heavy destroyer. those two units should never hang out together mainly because they are completely at odds in their battlefield roles Heavy destroyers/ Destroyers should always be moving away from the enemy, sniping at them from a safe distance where as destroyer lords should be looking for backfield non dedicated combat units to attack to disrupt the enemy battle plans. This of course could have been made more readily apparent by Games Workshop, but such is life.
Also, I'm curious how you came out to 9 kill points in a 1000 point list.
As for mistakes I made when I first started playing...
I started right after the 4th edition Tyranid codex came out, I was notorious for doing stupid things that the codex didnt allow, like give my warriors leaping and wings or my hive tyrant wings and extended carapace. I always got in trouble for giving my units upgrades that werent allowed. Also I had this nasty habit of attacking vehicles with my hormagaunts, for some reason I always forgot that they just simply couldnt do a damn thing to that rhino over there, but I sure as hell was gonna try. Also I loved sending my flyrant out ahead of the rest of my army completely solo... I could never figure out why he always ended up being the first thing that died. Ahh, the innocence of youth...