Still 250m swim

Second swim training today went a bit better than the first, Still only 250 metres but I felt a bit more streamlined today, but not much. My heart was pounding out my chest. I think I am trying to go too fast but with poor technique it is very hard work. I cant seem to keep my body straight my legs are sinking then my head comes up. I found a swimming DVD to watch. It is an Eight week swim program by Dan Bullock a former national elite swimmer and Swim For Tri founder. The DVD came with 220 Triathlon magazine. Ill be watching this tonight and hopefully see what I am doing wrong and how to fix it.