how do i create custom error page in config. and do i actually need to create one when i uploaded my site to web, it keeps

I.m very new in this web devlop. and i just finished set one website with membership required. when i uploaded this to web. it keeps displaying the same error saying "error mode= off" something like that. and create a custom error page in config. i'm so lost, i don't know what to do. now, my web hosting co. fix it where error displays window fixed error message " 500" don't know what that code # means. also whenever i try to log on my memberpage and type my userid, password, which it worked on visual web develp. but it's not working on the internet. it shows the same error message. i need help.