How do black hole suck up?

A black hole doesn't literally suck. A black hole pulls things closer to it. And it does this the same way that we stay on the earth--- gravity. A singularity, a point with mass but no height, width or length is at the center of every black hole. This singularity is what has the gravitational strength to pull everything, even light, towards it. It does it all with an unfathomably strong gravitational pull.