Random shoe knowledge:
Infant sizes run 1-12 with no half sizes. Children's shoes start over at size 1-6. Adult shoes run from size 6 and up.
Anything over 12 is considered an uncommon size, and typically does not come in half sizes. Most major brands jump straight from 12 to 13, 14, 15 etc - but no 12.5, 13.5, 14.5, etc. Shoe stores usually only keep 1-2 pairs of any given shoe in a size 12 or larger. Anything above 13 is extremely rare to find on the shelf, and usually has to be ordered.
If you wear a size 6 or smaller, you may need a kid's shoe. If you wear a size 13 or larger, you may need to special-order shoes. Rack-Room only goes up to 12. Payless often goes up to 13 or 14. Many major brands can special-order larger sizes. Half sizes above 12 do sometimes exist, but are rare, and often bespoke. Given the lack of half sizes common above size 12, it is often necessary to either find a 12 that runs big, a 13 that runs small, or to just lace the 13s up tighter.
Most men's shoes come in a D width, while women's shoes tend to come in a C width. Women's shoes are often sized 1 to 1.5 sizes different from men's. Someone who wears a size 9 in men's would usually wear a size 10 / 10.5 in women's. This does not apply to kid's shoes sized 1-6.
Not all shoes are sized the same, and it's common to need different sizes from different brands. Nike tends to be cut narrow compared to other shoes. New Balance has the option to get shoes in both narrower or wider widths. Skate shoes are often cut wider.
I wear a size 13 extra-wide. I'm a size 13 by length, but a size 15 by width. It is not easy to find shoes if you are above a size 12.