Keep calm. Try not to react to your child's outburst. Try to soothe him or distract him. If that doesn't work remove him from the situation for a short period of time to help him wind down. The second often forgotten part is to acknowledge good behavior. Remember: "negative attention is better than no attention at all.".
Behavior. Temper tantrums are normal for certain ages. The AAP had a good advice: http://www.healthychildren.org/English/familylife/familydynamics/communication-discipline/Pages/Temper-Tantrums.aspx However, there gets a point when they should outgrow these. Sometime, it's because they haven't been dealt with properly. It can also be more that that, and should be evaluated by your doctor. .