How do I install ubuntu server system from a usb stick?
First you need to download the Server ISO that you require. Go Here to get either the 32bit or 64bit 12.04 LTS or the 64bit 13.04 version. Once you get that done, I recommend that you download Yumi to create you bootable USB stick. Once created, make sure that you switch you BIOS to boot fro the USB stick, and just follow the steps.
Note: With Yumi you can also add other distros to either install or try, as well as other utilities.