How do I create Custom Mobs, Items, and Recipes?

[rigid flint]

I'm trying to make a server, and I figured I could just create the mobs first, but I have no idea how. Preferably, I can just dress up some mobs to look like the ones I'm mimicing and just re-use already existing items

[tidal cedar]

yea, im sure thats the way i would use items like fish or rare unused stuff