Well fitting, appropriately laced running shoes are a must. It helps prevent injuries and unnecessary pain (foot, ankle, knee, hip, lower back).
Go to a speciality store. The sales person should watch you walk and run to analyze your gait. Depending on your arch you may still need inserts. I personally have to special lace my shoes and usually have to get inserts as I have a wide feet, thin heel, and high arches...it makes shopping difficult as I simply cannot wear New Balance or Reebok. I usually get Asics gel Kayano, though they usually have some less expensive models that are fine as well. To save money, try looking at last years models. Many times the only difference is color or slight variations to the shoe.
Depending on how much you run, you should replace them every 6 months or every 300-400 miles. Even if they look fine, the cushioning and support breakdown. You can keep running shoes good by wearing them only for running, taking the time to untie and loosen laces before slipping your feet out, NOT RUNNING THEM THROUGH A WASHING MACHINE.