You should have an arma_server.exe in your install dir somewhere. Simply run that and you'll have the basic dedicated server. For more robust stuff, you create a server config, which allows you to set various bandwidth settings, password, MOTDs, etc. Your player profile ends up being used for the server difficulty settings as well.
Is the dedicated server free or does it require a license so you'd basically need to own two copies of the game to play on your own dedicated server?
EDIT: this dum dum answered his own question just by looking at the wiki page... The server file in the directory requires a license key and the disc in the drive(assuming if not steam) but you can download a standalone server. It says you need the DVD though so that makes me wonder if you can't have a standalone dedicated server if you have it on steam. That true?
I also had some pretty specific questions about what sort of load ArmA 2 puts on a server, and what sort of monthly bandwidth/cpu usage it can be using up to, was looking at switching hosts under the new load we have been experiencing. Is there someone specific I should PM?