To find the right size, you have to know what size you're looking at on the item page.
Is the sizing you have in front of you in
- European (usually a range between 35-45) or
- US (usually a range between 5-15)
I will show you how to convert it, so you can read it in a size you're familiar with.
So what's the size unit you have in front of you, and for what gender?