When creating a server, you have to look at how much traffic you expect, and what kind of server it is. Since you're making something like dropbox or mediafire i presume, the one major thing you'll want is space, or HDD. Heres a break down:
Servers are just like computers, they need all of the basic requirements of a computer, minus a monitor (that is, if you dont want to look at the console logging)
A BASE
You need a motherboard to be the hub for all of the components.
HARD DRIVE
Every server needs space, either Hard Drive (a write/read disk) or a Solid-state Drive (SSD). For fast computing, SSD would be best, but for most space, HDD would be best.
PROCESSOR
Ever computer needs a processor, this is where the computing is done. An important note when running a server is to have a multi-thread processor, meaning more cores than speed. This allows mutliple clients to connect at their own speed, and distribution is handled better. So an 8-core processor with 1.2Ghz processor is better than a 4-core processor with, say, 3.0Ghz.
RAM
A very important part of the server is its available RAM. This is what moves all of your data to and from the space drives, so the more you have, the more transferring at a faster rate. It goes in depth with the types of DDR being used to allow faster transferring and usage.
VIDEO CARD
This isnt important, but you will need a way of displaying the server to you, so any cheap display card will do. If you use Lunix, however, display cards won't be needed, but you will need a monitor to use the Linux console.
NOTES:
Running a server on a Linux machine is a ton better than one on Mac or PC, as it utilizes all of its components to work together, it has good optimization, but you will have to learn how to use a Linux console.
You'll want a DNS and a static external IP address if you want to run a web-based server.
If your server picks up a lot of traffic, you will need a large amount of internet to satisfy the needs of the clients, so be prepared.
You will want to learn about server ports and routers, because there are also info you can learn about optimizing a server.
Again if the server gets a lot of traffic, you may also be at risk of DDoS attacks ( Denial of Service/ Direct Denial of Service) so you want to learn a lot about routers and internet security.