Two options that spring to mind.
1st option:- purchase Snow Leopard from here:
If you have money to burn you could purchase Snow Leopard Server from here:
Although there would not be much point IMO?
Regardless of what you decide, once purchased insert the DVD, wipe the drive and install Snow Leopard. Once installed run Software Update and upgrade to 10.6.8. After the reboot launch the App Store App, create an Apple ID (assuming you don't have one) and purchase Lion. After installing Lion launch the App Store App again and purchase Lion Server.
2nd option: purchase the OS X Lion Thumb Drive from here:
Insert the thumb drive and reboot the Server hold down the alt key, select the thumb drive and at the setup assistant stage wipe the drive and install Lion. Once installed and the server has rebooted itself launch the App Store App again and purchase Lion Server.
There are other ways of doing this but the above should get you to where you want to be?