First, just a question out of curiosity: are you aiming to be a Volusia County Lifeguard?
But as far as breaking 10 minutes for a five hundred, here are a few tips:
1. Pure mileage, well "meterage" will help you improve.
2. Breathing. Obviously if you're on this forum, you're a runner or at least semi-understand what it takes to be a runner. What you're probably used to is fluid, natural breathing. Exhale, then almost immediately inhale.
Swimming is similar but not enough. The trick to breathing for swimming is to take a breath every third stroke (so that you'll be breathing on each side, i.e. bilateral breathing). Sometimes it can be too much coordination to handle if you've never swam, so it's easier to breath on the same side each time--EVERY OTHER BREATH. Start there and work up. And when you breath, don't gulp like it's your last living breath. Make it natural, and exhale while your head is underwater to save time while you turn to take a breath out of the water. The only thing different than running is adjusting to the brief lack of oxygen.
3. You can do build sets as well to increase your resistance. Assuming you're in a 25m pool, start with a warm up then do a 100, 30s break, 200, 30s break, 300 etc. Or you can always decrease or increase the intervals.
4. Hypoxic sets. You can increase your aerobic capacity by doing a series of say 10x25m lengths. For half of that drill, don't breathe (12.5m).
And on a last note, if you're getting out of breath after 2 laps, cool it down. Don't swim so fast. You'll be able to build up to 500 if you don't wear yourself down so quickly.
If you have any other questions, post back. I've grown up a swimmer. I can't run for the life of me, but my running-nut boyfriend saw this and asked me to answer.