Well, as someone who has had 1 hour of private coaching and then made the rest up as I went along, I'm obviously a fully qualified expert
My general tips are these:
1) Start off at a pace that you think you can maintain for whatever distance you are trying to cover. Then adapt/adjust it as you go along. You'll eventually get into a natural rhythm.
2) Breathe more often. There are no points for drowning. The coach I saw who specifically coaches triathlons told me that I should be breathing more than every 3, which is what I was also doing at first. Then again, this was always with a 1.5km swim in mind. I now breathe every 1 to 2.
3) Your head is heavy. Don't lift it out of the water (apparently this is a very common problem), just twist it and kind of gurn your mouth to one side to breathe.
All the other things he told me were specific to how I was swimming (1) I was spinning my arms like a windmill rather than a T-1 stroke, 2) I was not lifting my elbows out of the water very high as I wasn't twisting my shoulders properly, 3) I wasn't doing the "catch" as I was trying to do the now antiquated thing of making a sort of "S" shape under me with my hand).
Thing is with swimming, once you can swim, well, err, you can swim. But you are probably doing it very inefficiently. To get any good and not kill yourself is all about technique. You can read all the theory and *think* you are doing it right. Then someone will watch you and you'll discover that what you thought you were good at, you probably are not. I would highly recommend getting some coaching early on - it is far easier to start slowly and get it right from the word go, than to try and correct bad technique. I did my coaching session in an endless pool so you swim into a fan and they video you from every angle. Suffice to say I was not the mermaid I thought I was. However, he did get my strokes per min down from 58 to 46 with only 1 hour of coaching. So now I swim at the same speed but am far less knacked at the end of it
HTH. Let us know how you get on