Snowboarding boots, what size to wear?

i want to try out snowboarding instead of skiing, but heres my story. My brothers friend has a good snowboard for me to try out with bindings, but he doesnt have any boots for me to use. So i wear around 7-8 everyday shoes, and im looking for some snowboarding boots to buy online but i dont know what size i would wear in boots... i heard theres soem kind of size changing operation or something :D Thanks in advanced for your lovely answers