Are you dreaming about your first triathlon? Stop dreaming, you have to start now because every day counts! First thing's first: find a sprint triathlon (short distance) in your area that is at least 6 weeks away and sign up. Now you will need a training plan. Here is a tried and true 6-week training plan for beginners that will get you from day one to the finishline.
Monday: One hour bike ride, flat, easy
Tuesday: Swim training 1km (200m free/ backstroke, 10x 25m free fast with 1 minute rest, 200m free/backstroke, 100m kicking, 200m free, 50m backstroke)
Wednesday: Recovery day
Thursday: 30 minute run (walk for 5 min/run for 5 min)
Friday: Recovery day
Saturday: One hour bike ride, flat, easy
Sunday: Swim training 500m focus on correct technique