I am 18. I have a small painless bump on the right side of my neck. Could it be tumour/cancer?

Unlikely. It is unlikely that the bump is cancer. Check every 2-3 weeks, and if it becomes more than twice its current size, see your doctor. For good health - Have a diet rich in fresh vegetables, fruits, whole grains, low fat milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco or alcohol in any form. Practice safe sex. Get HPV vaccine.