Mod utilities and resources
Your first stop for Dragon Age modding is the BioWare Social Network, where you can find the toolset, tutorials, discussion pages, and of course mods themselves. For many this will serve perfectly well as a one-stop shop, but Dragon Age Mods and Dragon Age Nexus may provide additional useful projects and information.
 The toolset
The toolset is the only necessary component to modding (though of course external tools can be applied as well); it can be downloaded via the helpful links section.
Mods are typically easy to install thanks to the DAContentUpdater app located in your C:\Program Files\Dragon Age\bin_ship folder -- just drag and drop. After that you can enable or disable mods through the DLC menu in-game, or access campaigns through the "Other Campaigns" menu option on the main menu.