I will be going above 15000 feet (4500 meters) on a trip next month, and I've heard people will often get altitude sickness. Is there anything I can do beforehand to prevent sickness or take while I'm there that will help me.
Are you going to be hiking that high, or in a car for a small bit? Going to 15,000 feet without ever having broken 10,000 feet sounds a bit haphazard, especially if you're unsure of the dangers/how to deal with them.
There are several things you can do to help yourself out before your trip.
First, acclimate. Don't just run up to 15,000 feet. Try to spend time at 8,000 feet, 9,000 feet, 10,000 feet and so forth. Additionally, climb high, sleep low always applies. If you're not feeling well, descend, it's the only way you're going to feel better.
From a chemical perspective, diamox can be prescribed by your doctor to help with acclimatization. I've used it above 13,000 feet and it's greatly helped.
Stay hydrated, you rapidly lose hydration at altitude. Tea is your friend, so bring a good amount of it.
Last time I was at 18,000 feet, we chewed ginger root and it helped to some degree.
The best thing you can do is acclimatize. This means you should adjust your body gradually to the height. This can be done, for example, by increasing the height you're staying at from day to day. Another very important fact that is widely used by mountaineers is that you should always sleeps some meters below the highest point of the day. So for example, you reach the height of 4000 meters, where you plan to spend the night. Before going to sleep you should if possible gain another 200 - 300 meters, spend some time there, then go down and sleep at 4000 meters.
A lot of mountaineers also use Acetazolamid (Diamox), which can be taken as a preventative. But its use is discussed controversially because it has a lot of side effects.
Additionally to that you should drink a lot, but no alcohol, and you should also take care of your nutrition. It is very important that you force yourself to not use a lot of power. (This is particularly important when climbing a high mountain. You have to force yourself to walk slowly enough). If you feel slight symptoms it isn't necessary to do anything, but if it gets worse you should go down to lower altitudes as soon as possible. If it is really bad, you should immediately seek medical help, because it can kill you.
Here is a reliable medical source of the causes, symptoms and advised medical responses for altitude sickness of varying degrees. Cleveland Clinic - Altitude Sickness information
Altitude Sickness Explanation
Guidelines For Proper Acclimatization