There is a ton of stuff on youtube and elsewhere on how to throw punches, some of it better than some others, none of it very good, in my opinion, but I'm picky that way and its a place to start. In general, try keeping your feet a bit closer (front to back) than what they demonstrate in these videos: turn your hips into your punches more than is shown: throw your left hook with the palm inward and the thumb on top.