Make sure you have enough room in the toe. There should be roughly one thumb width between the end of your foot and the end of the shoe. Your foot should be centered on the shoe platform with the shoe wrapped comfortably around your foot, not tight and not sloppy.
After you found a good fitting pair, try them out. Run on a treadmill or do a few laps around the store to make sure there’s no irritation or slipping in the heel – which could lead to blisters. Also ask at the shop about good running socks – they can make a big difference.