Measuring the length of your feet is the easiest and most accurate way to determine your proper shoe size! Just measure your foot from heel to toe as shown here:
WHAT DO I NEED?
How To Measure Feet:
Alternatively you can examine some sneakers you already have.
Shoe size can be easily determined based on the brand. For example, if you plan to buy Adidas sneakers take a look at Adidas sneakers you already have.
Brands have their own molds that are the same. The molds of other brands are only slightly different.
Finally you could visit a nearby store and try sneakers similar to those you plan to buy.