Get into shape. The physical demands of being a lifeguard require that you be in excellent physical condition. If you are not in good shape, the strenuous tasks involved in a guard course will be overly stressful for your body and you risk injury.
Find a lifeguarding course. Courses are offered by the American Red Cross and the Young Men's Christian Association (YMCA), as well as a few other places. Check your local listings for these agencies or try your local gym to see if the course is offered there.
Determine the prerequisites for your course. These should include a basic first aid class as well as CPR certification. If these are not prerequisites and are not covered by your class, take them anyway. Lifeguards need to know how to bandage knees, care for bee stings and a myriad of other summer hazards. Take the classes and be prepared to handle these and other similar situations.
Take the course. You are now prepared to get a lifeguard job, but if you want additional training, check with the American Red Cross and the YMCA for additional and more specific courses.