Don't worry; we've all been there. As I recall it, my first dive was about 15 minutes before I got to 500 PSI. Now I can get to an hour and a half with the same air. How do you improve it? Lots of ways...
1) Diving a lot will get you comfortable, which makes you relax, which slows your rate of breathing.
2) New divers tend to swim too fast, racing past all the good stuff. Diving is mostly a slow, easy sport.
3) Make sure you are warm enough ... if you are cold your body will need fuel to warm up, and the oxygen in the air is that fuel.
4) Make sure you are not over-wieghted ... new divers seem to be consistently 10+ lbs over weighted because instructors, rather than taking the time to teach proper descent techniques, just tack on more weight to get students to the bottom to go through the skills.
5) Make sure you are neutrally bouyant ... you should be able to hover in place without finning. Can't tell you how many times I've seen classes of divers furiously finning past, leaving a huge plume of sand in their wake, because they were so over weighted.
6) Don't waste air on the surface: Use your snorkel when on the surface. Orally inflate your BC rather than relying on the auto inflator.
7) Make sure you are aerobically in shape. Scuba diving was the impetus for me to start spending time in a local pool doing work-outs. In addition to losing a few pounds and changing my % body fat (remember, fat floats), I got used to exercising and recovering in the water.
8) Some divers resport to buying a larger tank (a steel 100 seems to be most common) but I've never been a big fan of that. Usually after 20 or so dives everything gets easier.
Be patient ... it'll improve with time and experience.
