Nice start to the story. Keep writing.
There's a lot of backstory begging to be filled in: how Harry managed to defeat (killing with a rock is fairly easy once you've diswanded him) Lucius, when he slept with Lavender, how is his father's friend who taught him sex, who else beside Lavender he may have enjoyed.
Since killing Lucius was "ridiculously easy," I am imagine Lucius apparated into the backyard while Harry was working in the garden. While Lucius was marching toward the house, Harry grabbed a nearby rock (perhaps that was even his chore, gathering all the rocks out of the garden, so he had a pile handy) and rushed up to the unsuspecting Malfoy and clobbered him.