What causes the Earth to have gravity? I've always wondered how our planet Earth or any other large spheroid-shaped thing in our universe has gravity? 

Does it have a small black hole on its core causing it to have gravitational force?

Take our planet as an example: what causes it to have gravity?
Any massive body (i.e., a body having mass) like the stars, planets, moons, and any atom in the universe have, what is called, a gravitational potential which results in its gravity.  This is described by Newton's Law of Gravity. Why masses cause gravity, we do not know.  

So, in short, the earth's mass causes it to have gravity.