Generally, senior hockey skates fit 1.5 sizes down from a men's shoe size while junior and youth hockey skates fit 1.0 size down from a boy's shoe size. For example, a player wearing a size 9.0 men's shoe size would select a senior size 7.5 hockey skate. For junior or youth players who have quickly growing feet, selecting a skate only ½ size down is an option to allow for extra room for growth.
When judging what size you need, make sure you are wearing the socks you'd normally wear while skating.
Hockey skates sizes vary between manufacturers so you should always check each product page for that specific skate's sizing information. Generally speaking, a vast majority of players should fit well in a Medium Volume Fitting skate in the Standard D, R or E width.