The most important item for any runner is a great shoe. With the wrong shoe you can cause all sorts of problems from blisters, to plantar fasciitis, to calf pain.
In order to determine what type of shoe best fits you, visit a running specialty store in your area and have an employee in the shoe department watch you run. Depending on whether your foot rolls in, out, or doesn’t roll at all, you need a specific shoe for your foot type. If you purchase a shoe from a specialty store, they usually have good return policies and will let you return the shoe even after worn. I worked at a local running store for a few months and saw several people bring back shoes with blood stains from blisters.
Once you know your foot type (I slightly overpronate), use an online store such as http://www.roadrunnersports.com/ to purchase your shoes at discounted rates, for future runs. I buy several pairs of the same shoe at once, and recommend you do so as well. When you find a good pair, stock up! Another good site is http://www.runningwarehouse.com/
It’s important to change out your shoes between 300-500 miles. I throw on a new pair before I begin a training for my next marathon. A little tip, when you do buy a new pair, don’t toss your old ones. Keep the old pair as “rainy day” shoes. This way your new shoes won’t get dirtied with all the muddy water in the streets and along the trails.
When you do finally decide to toss your shoes, be sure to swing by the the same running specialty store to donate your shoes, no matter how old. One man’s trash, another man’s treasure. (I wear the shoe in the above picture for my training runs, the Asics GT-2000, and for races often times wear the Asics DS Trainer as it is more lightweight)