Young, welcome to the Tri community! Envy you for starting early. I started last year at age 64. I swam in my home pool (30') and thought because I could ride 100 mi easy that swimming and running would be a snap. WRONG! I joined a local master's swim class and found I was struggling to do 25 yds. Last Sunday I did my first Ironman event, IM Augusta 70.3, which included a 1.2 mi swim. I came out of the water 2nd in my Age Group, a position I held until mile 62 in the race. At mile 65 my hip acted up, and I had to walk the last 5 miles. But I enjoyed every minute despite the pain!
You are getting the benefit of lots of good advice from Old, Lud, iBike and the others. I've never used a snorkel for my swim training, but like other swim tools (short fins, pull bouys, hand paddles, kick boards) these can have a great effect IF you know what you are training for, and they don't become a crutch that you depend on. (You can use a snorkel in most Tri events, but you will not be eligible for any age group awards. That's how much keeing your head down in the water improves you swim techiques)
More than any other Tri event, technique is the key to success in the swim. Last October (2012) I couldn't swim 25 yds without being totally pooped. I'm now confident in my ability to swim a 2.4 mi Ironman swim. It will be slow compared to many, but I'm confident I'll be near a podium finish. And that's not because I'm that great of an athlete, but because I take instruction well, and I've focused on TECHNIQUE and EFFICIENCY not speed. I've subscribed to several PODCASTS on triathlon, like Endurance Nation, TRI SWIM COACH, FIT FAT FAST, and a host of others. Why not learn from others who have already made the noob mistakes?
Even though I was last in the water last Sunday, I found my self swimming over others who were swimming hard and inefficiently. I focused on a long, easy stroke, and keeping my body streamlined. If you have read about efficient pedaling on a bike, there are some keys to thing about in the pedal stroke to ensure you are "pedaling in circles" and not "mashing" Like pedaling in circles on a bike, an efficient swim stroke can be envisioned in a similar way.
I imagine my hand wiping fish slime off my palm on my hip as I bring my trailing had up out of the water. I drag my finger tips in the water, with my elbow pointing toward the sky. My hips have rotated so my shoulder, elbow, and hip are on the same plane, about 50 degrees skyward. As my had passes near my head, my pointed fingers plunge into the water about 8 - 10 inches in front of my head, and try to "spear" a fish about 3 feet in front of me. DAMN! I missed, but I reach to grab him with my cupped hand. I fail, but in desperation I grab a hand full of water, hoping to move closer the the elusive fish. Trying to be quiet and sly I pull myself forward, my hand not very deep. I imagine I'm crawling, trying to sneak up on the miserable fish. My hand accelerates close to my body, elbow bent and near the water surface, and I realize I have fish slime on my hand again. I pull it close to my hip, and wipe it off.
Meanwhile the same set of actions have happened with the other hand. If you are kicking, even a small amount, and keeping your head down in the water, you will be effieiently slicing though the water. As I mentioned, a snorkel will allow you to follow these steps with out having to worry about breathing. But once your body is trained, you need to learn to swim and breath properly.
A kick board can help here. Short training fins which float will help provide butt lift and speed that will give you confidence in your swim stroke. Find some training plans on line that meet your time allotments and fitness level. Follow them as close as possible. These allow you to build confidence and stamina, but THEY ARE ONLY TOOLS. You cannot depend on them in the long run. They are just training wheels.
IF you can afford it, HIRE A SWIM COACH. Even if it is in a group class. That's what I did. What I lost in 1 - 2 - 1 coaching advise I gain 10x in encouragement from the others in my class. YMMV.
Anyway, like Nike says, "Just DO it!"
And good luck!