The Koopa Troopa is very simple, basically you just need to make an NPC that turns into a koopa shell when killed. If you don't know how to do that, then you can watch this
viewtopic.php?f=101&t=23739. If you're completely new to lunalua, then you should watch the whole series, but if you know the basics, then you can just skip to the NPC series. If you still have any questions, then ask them here.