A dedicated server is a single physical computer engineered to support multiple users, run a large number of different services and applications, and manage, store, send and process data 24-hours a day. A dedicated server allows for all the resources of the physical computer to be "dedicated" to one client and the hardware resources are not shared with any other clients. This is in contrast to shared servers and cloud servers where the resources of the physical computer system are shared amongst many clients.
The answer to this question largely depends on two factors. Are you new to server technology and not very interested? In this case, Windows is the easiest option to get where you want to go. Do you already have experience with Linux or would you like to familiarize yourself with this operating system? If so, Linux gives you more freedoms to configure your server, and you will be using an operating system that is less vulnerable to attacks from the Internet. Linux is also open source and therefore free.