For making content (Dungeons/Items) on a RotMG private server, only C# syntax and behavior scripting is required to know I'm pretty sure. Since a source such as Fabiano Swagger of Doom or Seraph's Dominion already has the base code already coded, all you need to do is add content. I would start by watching/reading a tutorial at least once. I recommend on reading/watching tutorials in this order: items, maps, and dungeons. Maps and dungeons are probably going to be on the same tutorial though. If you just want 1 map for your dungeon, you are able to just create a map in the map editor. The hardest/most entertaining part is writing the behaviors for your enemies. There really isn't much of a tutorial on this other than looking at the different behaviors on the source. FSoD has the most advanced behavior logic for public servers I'm pretty sure. Just go to the Tutorials/Source Code sub-forum for the tutorials, they shouldn't be too hard to find. Most tutorials are labeled: [AS3], [FSOD], or [SD]. When you see a thread with the label [AS3], 90% chance it will work with Seraph's Dominion.
Summary: If you want only content, download Fabiano Swagger of Doom (download is in ossimc82's release thread or ******) or if you want to learn how to code in C# and AS3, download Seraph's Dominion.