If you don't know how to swim crawl at ALL, then I'd recommend searching youtube for some guides.
Look at this one for example. This is not a very good guide in my opinion, as you can see where he's maxing at the and of it, and his hands and feet are like a washing machine..
You should keep your body straight, breathe every 3 armstrokes, and keep your head facing down towards the black stripe.
Also, keep your legs underwater and always straight. For the armstrokes, do loooooong and sloooow; but do them good and use a lot of force. As I said earlier, gliding is key.