Snowboard boots are sized just like shoes, so if you wear a size 10 skate shoe, go with a size 10 snowboard boot. Yes, it is that simple.
A few things to consider about boots are performance, comfort, stiffness, and lacing system. A stiff boot will offer better performance, but isn't the best choice for beginners. A more comfortable boot might be too soft to handle advanced riding and high speeds. The stiffer boots often have an inner boot, which laces independently from the outer boot to give added support.
Most boots have a standard lacing system, but several of the boots we sell have a Boa closure system. This speeds up the lacing of the boot and gets an even snug fit in seconds. It is incredibly easy to use, but may not give the tight fit that many advanced riders need.
Snowboard boots first started out as Sorel-style boots, usually wrapped in duct tape. They offered a lot of flexibility and almost no support. That was when we didn't know any better. Boots have come a long way and are better looking and actually improve the riding experience.