Take measurements from heal to toe while standing, and wear the socks that you plan on wearing with your new shoes.
Use these charts to measure your foot and convert into a shoe size. They also help convert your US size into Euro and UK sizing for certain shoe manufacturers.
Note: These measurements are only approximations for the size you will need. They do not take the width or depth of your foot into account. Also take into consideration that styles may vary in fit based on the construction and manufacturer.