Train, train and more training. How much training really depends on what distance the triathlon is. It is much better to start with a shorter distance first eg 400 swim, 10km bike, 3km run. In preparation it is very important to spend time at all three activities namely swimming, running and cycling. Joining a triathlon club is a great way to prepare because they often run organized training sessions and invite participants of all ages. Check the internet and ask at running/sports store about local clubs.