Mod idea - Need help

Right I have a decent mod idea, I'm not going to go into any detail until I am allowed to put it up on here just in case someone tries to steal it :') (Sorry I am quite paranoid). However I will upload one image to give a little taster. But yes I do need some help... If anyone is willing then please don't hesitate... What I need is to understand how to make my created mobs hold certain weapons... and also how to put some constructions I've created into the game... Oh yeah and how to make a mob attack other mobs that are 'different' to it.

Thank you to anyone who helps me ^-^