1. Take Some Deep Breaths
This is easy and effective. Deep breaths will calm you down and make that big stressor seem much smaller.
2. Get Yourself Laughing
Anything that makes you laugh (a tv show, youtube video, etc.) will get your mind off of that stressor. Also, laughing will put you in a much better mood.
3. Talk To Someone About Your Anxiety
Sometimes, keeping everything bottled up creates even worse anxiety. If you tell someone, like a friend or parent, what you are stressed about, they might be able to rationalize it, or make it seem less scary.
4. If It Is Specific To One Task: Just Do It
Who knows, maybe you will even enjoy the task at hand. (But even if you don't, it will at least be done.)
5. Get Help
Sometimes, anxiety won't go away without professional help. There are probably a lot of wonderful psychologists, therapists and psychiatrists who want to help you out.