One of my biggest pet peeves is when people wear shoes that are not good for their feet, both while working out and in general. A good pair of running or walking shoes is something that you MUST invest in. You are going to save yourself a world of hurt in your legs, back, and your entire body, prevent minor to serious injuries, perform better, and, based on all of that, improve your mood and self as well.
So, find a shoe store near you; I find the best results are stores that specialize in running, or some type of exercise, or orthopedics. In the St. Louis area, this includes Big River Running (http://www.bigriverrunning.com) and Fleet Feet (http://www.fleetfeetsports.com/). There you can ask the person working to examine your foot, explain what your daily activity level is that you need your shoe to support, and they will help find the best shoe for you. Don't worry, despite how much the worker wants you to, you do not have to buy the shoe there, especially in a specialty store where the shoe may be more expensive. Just remember the make and model of your shoe, do your research, find a pair you can afford, and then enjoy the gift you have just given not only your feet, but your entire body!
Here's a website that talks about shoes bad for your feet: http://fitbie.msn.com/get-fit/tips/4-shoes-wreck-your-body
When I decided to train for the Chicago Marathon a few years ago, the guys at Running Wild in Coralville, IA (http://runningwild-iowa.com) fitted me in the ASICS Gel Cumulus (http://www.asicsamerica.com/), as opposed to my usual Nikes I wear when sprinting. Many of my long-distance running friends have been fitted with this shoe as well. Love this shoe!