I am having severe pain on the outer top part of my left foot. I can't walk on it as when I tried my ankle started hurting and now my whole foot is swollen a little.. what could it be?

What you describe would typically be from injury, so can you think of any?
A sprain/twist/hit against something? Are you a runner/jogger?

Ice on swelling, heat on pain when swelling goes down.
If pain persists, you should have an X ray to see if a bone chipped. A tiny chip won't be treated, but it will hurt.

I fell down some steps once, and the pain was excruciating until I pressed all over my foot and a huge bruise came to the surface. The pain was gone instantly, even though I had a very purple and swollen foot.

It being the top like joy said an injury and your ankle hurts from compensating your balance most likely.