Amigo, it is a bit more to it, proper "Server" is not just PC. You need to be more specific.
Hardware spec will depend on many factors eg.
What is your intended use for?
What OS you envisage using, Windows Server or Linux based Server, Licensing ?
What you intended to host on the Server, files, printers, music, video, feed?
Number of users?
For hardware, you need to spec, Motherboard, CPU and memory to match.
Storage, HD's space, chassis, power supply, UPS, backup, any redundancy eg dual power supply,
mirrored disks, array RAID disks etc and most important, what is your budget $$$$$
Obvious depends on your requirements and application some of the above may not be that important
ps yes you can run laptop as a simple server, BUT ???