It could be something as simple as your shift/brake cables rubbing on the frame or it could be your headset bearings. You can quickly check for a loose headset (one cause of a creaky headset) by grabbing the front brake tightly and rocking the bike back and forth. You should not be able to feel any play or knocking as you do this. The frame should only move as much as the fork does.
If you can feel knocking, you'll want to adjust your headset ASAP. You could ruin your frame/fork if you ride too long with it loose. Given that your bike is only 5 months old, it should still be covered under warranty at the shop you bought it from. If that's not an option and you are somewhat mechanically inclined, adjusting a headset or regreasing the bearings is a pretty simple procedure though it does require some skill in the bearing preload adjustment. Read up at
www.parktool.com