You have dry skin, which increases more in winter. You will get relief in summer; and you can use moisturising creams containing vitamin E and aloe vera.
Dandruff is a common problem. You need not worry; you can use antifungal creams containing ketoconazole. Use in 1:1 dilution on alternate days. If partially controlled, use twice a week. If controlled, maintenance use is once a week.
Red spots are due to prickly heat - do not scratch otherwise it gets infected resulting in blood & pus discharge. To prevent prickly heat you can use nycil powder.
Regarding oil you can use coconut oil for scalp.
Regarding diet there is no relation to your problem. It is a myth. You can choose any diet of your choice. You can eat chocolates, which will not increase your problem.
You needn't be anxious, as these are common problems anyone can encounter.