Stop thinking about stress! This is easier said than done. Our minds are not very good at processing double negatives - try not to think about a big spotty, purple cat and notice what happens.
Instead of trying consciously not to think about stress, think about something completely different. What could that be? What else could you do? Forget about stress, think instead about something positive, something very pleasant, your goals and dreams for the future. Think about who you will be and what you will be able to do without ever present stress in your life.
Stress is a medical condition and it can have a serious effect on your health. If you think you are suffering from stress it is crucial that you consult a medical professional.