mooncalf - try just standing in the pool and bending your knees. Get used to the feeling of your head being under water before you try to do it while swimming - it makes a HUGE difference when you try to do crawl for the first time.
And then, rather than try do do whole lengths, try doing about 6 strokes, holding your breath, with your head under the water the whole time, then stop (you're best to do this when the pool is quiet, or in the blue rinse lane, so as not to annoy faster swimmers). It gets you used to the feeling without having to think about how to position yourself for breathing aswell, which often results in a lungful of water.