go to tumblr.com and then when it asks for a 주소 당신 are basically choosing a username. it's just that your 아이디 will be an essential part of people finding your blog
It's actually really easy. When they ask for the url, it means for //your// tumblr. So like, if your name is Demi, 당신 could make it demi.tumblr.com Get it?