My left foot itches as soon as I take my trainers off what Will stop it?

Diagnosing it and TX. Get a diagnosis, then treat it. (For example if a fungus, there are meds for that,etc).
Talc Powder. You probably have a mild form of Athlete's Foot (tines pedis), which is the result of having too much moisture in the shoe and around the skin of your foot. If you have no active breaks in the skin between your toes and itching is the main symptom, you can try applying over the counter Gold Bond Medicated Powder to keep the area dry. Try putting it in your socks before putting on your trainers.