Well first of all you have to relax a little more while swimming, most anxious/nervous swimmers will tend to inhale too much and not exhale enough(hyper-ventilate), common for your first few tri's. Focus on exhaling about 80% of your air while you are taking your stroke with your non breathing side arm and the remaining 20% as you are rolling from your hips to take a breath(your head should not be turning). Relax, relax, relax, that's all you really have to do in order to not gulp to much air, release what you have taken in from your last breath and you will be fine. Get in the water before hand and do at least a 5 minute warm-up, this will help reduce the shock you experience from the water temp, the nervousness will fade with experience. Good luck!

-Anthony