Toe box, heel cup, heel lift, cushioning, support
Look at your feet – an aerial view of the shape of your trainer should be a similar shape to your foot. – If you have a wide foot you will need a wide trainer and/or a wide foot box (bit where your toes go). If you have a narrow heel you will need a suitable heel cup to ensure your rear foot does not slide around.
Pronation is a normal part of gait that is both necessary and important. Your foot shape and skeletal structure will play a role in determining the posture and mechanics of your foot. Just because someone tells you that you have flat feet or that you pronate, do not be scared into trusting they know what trainers will suit you.
If you have limited ankle range of movement, have super tight calves, or spend a lot of your day in heels then you are likely to need some heel drop, going to a totally flat sole may cause you issues.
Does it fit properly and is it comfortable to wear? It should be totally comfortable right from the start.
Buying a shoe that fits and is comfortable and is specifically suited to your sport and body type is the most important thing.
Buy trainers which you find comfortable and designed to meet the needs of your event/body size. If you run sprints on track then buy light, spike designed shoes; if you run marathons spend the money on marathon shoes which have cushioned soles for running longer distances; if you are heavy set then buy something with lots of cushioning to try and help protect you joints. Whatever you buy…break them in gradually.