I would highly recommend going to somewhere like Runners Needs and have them look at your gait. They do this by filming your running style on a treadmill. This will determine the level of support you need based on how your foot hits the ground (flat footed, leaning onto one side or the other, or just normal), your stride etc.
Make sure you leave enough space round your foot for it to expand and ideally take the socks you wear and try them on with them.
If you are using your trainers for running then make sure that’s what their purpose is, as there are different styles of trainer for different exercise usage. Also think about the type of running you’ll be doing as you can get different ones for cross country v road running for example.
Walking about the shop won’t replicate what you do when you’re out running, so ask if you can use the treadmill for a few minutes to ‘test’ them out, before you buy.