When you go to a shop for boots, they will always measure your feet right there first thing. If you like, you can measure this yourself at home. Mondo sizes are based on the metric length of your foot. 28.5, for example, is roughly* 28 and a half centimetres.
Take a ruler or measuring tape and place it on the floor with one end directly up against a wall. Stand on it with one foot, push your heel all the way up against the wall. Measure to the end of the furthest toe. The starting point for boot fitting is usually the nearest centimeter rounded down(-ish).
*mondo shell sizes don't correspond exactly to the metric measurement for a few reasons, but it's close enough for explanation purposes.
The reason boot fitting is important is that ski boots fit much more precisely than casual shoes do. It's impossible to shop around for a boot online without knowing how well your feet fit with any given manufacturer's shell shape. They're all different and we all end up having to try a few in the store before the right one is found. And don't agree to buy a boot you have misgivings about in the store concerning the fit. If the fitter is rushing you, find a different store. This is the most important gear purchase you will make for skiing and cannot be rushed. Rushing leads to a poor fitting boot that you'll be whining about for the entire time you ski them.