First of all, consider why you're looking into a dedicated server. It'd help to know the reason as to why you'll need one, to help you decide what to look for.
Either way, if you've got no experience managing one, you'll either wish to get a managed service, or an unmanaged one and hire a server administrator to set it up, lock it down and configure it to your needs. Keep in mind that the latter option is not a one-off fee, as you'll need regular maintenance.
You'll further more need to gather how much resources you require for whatever your project is, and scale your server after that (optionally with a bit of overhead). Placement of the server will also be important, and should reflect your target demographic.
Finally, you need to decide whether or not you wish to lease a server, or purchase one and co-locate it in a datacenter. The latter is cheaper in the long run, but the former greatly widens your options and is much cheaper to set up.
Best of luck!