So I am a newbie of Plugin Development I just was finishing my PHP Course so I was wanting to make a plugin
To learn the PocketMine API, you can check out https://github.com/PocketMine/Documentation/wiki/Plugin-Tutorial and after that, just read some plugins' code and start messing around with your own code. You'll get the hang of it after awhile.
I recommend to also check out the resources here by other devs http://forums.pocketmine.net/forums/resources.44/