What Causes Pain And Swelling In Ankle After An Injury?

Hi, today I was skateboarding and when I fell my left ankle kind of rolled inward, and then I landed with my right foot bearing all my weight onto my left ankle. Most of the pain is in the left side of my ankle, but there is swelling and black and blue on the inside part of my ankle. There is a lot of pain when I apply weight to it, and even when I am just sitting down. How long should I was before I were to go to a hospital and get it looked at?