how to make a usb stick bootable for windows xp

For that you need to have iso file of Windows XP installed CD/DVD. Hoping that you already have one. You can either download the Bootable USB Utility from microsoft (it is free) or use third party tool like PowerISO and it will make bootable usb for you.
Here is a link from microsoft store. It will work for XP as well. Read the instructions:

http://www.microsoftstore.com/store/msusa/html/pbPage.Help_Win7_usbdvd_dwnTool