Hi kruzzen,
Well, plugins are written in a scripting language called PHP. Googling PHP tutorials should reveal a plethora of worthy guides on how to start scripting in PHP.
Have you any programming/scripting experience in the past? If so, it will help you greatly, and make things much easier to learn. If not, you may find yourself stumbling a bit over your first programming language. Don't worry however - this is common across all programmers (I think)
And once it clicks in your mind, then things should be much easier.
Of course, if you're having troubles, I'm sure others will be willing to help.
Oh, and once you learn PHP, making a MyBB plugin/modification should be fairly straight forward.
Good luck!