If you are intending to run a marathon, you need to be physically well-prepared and fit as it can be an exhausting and painful experience otherwise. Without working on your fitness level first, it can take a terrible toll on your body. The basic method of training for a marathon is simply to run and to run intelligently.
Other than a good pair of running shoes, there is no special equipment needed. Choose shoes for comfort and support. Shoes that give you a little spring in your step will not hurt. Carry a water bottle with you each day you take to the hills as it is important to stay hydrated.
Begin training with baby steps. Professional runners and trainers suggest giving yourself at least four months to prepare before the big run, training a few days per week. This will ensure you have the endurance level to complete it.
If you are training without a coach, you need to stay motivated and work every day on increasing your fitness level and endurance. Begin with short runs of a half-mile if you are a beginner, but work your way up with each passing day. Design a schedule that works for you. Take to the hills to experience those more difficult sections of a marathon. The goal of the schedule is to work your way up to 15 miles, but stagger the distances in those final weeks. Run just a few miles one day, one mile the next, a slightly longer run the next and one very long run the next. Coaches advise against running longer distances just prior to the event. You could be too exhausted to complete it even if you feel ready.
The key to success is willingness and motivation. You want to be fit and able to go the distance. Do not think of it as a race. Think of it as a job completed. Train intelligently and safely, eat healthily and you will succeed.