The best way to start training is to improve your overall fitness level. Start running every day if possible. This is the quickest way to kick start your conditioning. Then hit the gym. Here are great tips on starting your boxing training: http://www.livestrong.com/article/174395-how-to-start-training-for-boxing/

Core strength is crucial in your condition as the core is called on with every move you make during boxing. Follow this guide for core strength exercises.
http://boxing.isport.com/boxing-guides/core-exercises-for-boxers

Once get your fitness level up to par you will be ready to start learning the jab. The first punch learned by beginner boxers. Follow this guide and video on proper technique:
Video: http://boxing.isport.com/boxing-videos/play/the-jab-17969
Guide: http://boxing.isport.com/boxing-guides/basic-boxing-punches-the-jab

Good luck with your training!