Autodownload is a feature that lets you both make pk3's from your copy of quake3 available to download and played and to download pk3's from another
servers, mainly with Internet connection. It should be somewhere in game settings (try System)... If you want your
server be listed in Internet
servers, you need a publicly accessible IP address (which is not common with home Internet connections), also you need to set dedicated
server in your game configuration - this will shut down the game graphics displaying the console - do not be scared - this is dedicated
server. To play you just need to run another instance of game and connect to that
server using that another instance (even on the same machine). Most people run their
servers on hosting
servers for a fee to make it permanently accessible for players over the Internet. But it's not cheap thing, so think twice if you really need that. If you just want to play with only your friends, try to set up VPN with your friends and make LAN
server instead of Internet - that way you make that
server accessible for you and your friends only and won't be accessible for another players from worldwide. I play using VPN connection with my friends when it comes to that

Hamachi is good to make gaming VPN with friends
