Anyone who has bought a lot of different pairs of shoes will tell you that different companies have different interpretations of shoe size. While this might not be a problem for children who will eventually grow into a larger size, it is important once your feet stop growing to have a shoe that is not too big.
Buy a pair of shoe insoles that match your shoe size. If possible bring the shoes to the store and try different insole sizes so that you get the best fit.
Put your hand on the rubbery bottom of the insole and slide it into the shoe keeping the top of the insole in contact with the top of the shoe.
Push the insole all the way to the tip of the toe of the shoe and make sure that that heel of the insole is snug inside the heel of the shoe.
Lace up, slide into or buckle the shoes and walk around to get used to them to ensure the shoe is now snug, but not too tight.