To set this keybind, bring up World of Warcraft's main Options menu (hitting Escape will bring it up), go to the Key Bindings menu, scroll down to Targeting Functions, and you should be able to find the option easily. You can also set an Interact With Mouseover key bind if you so choose. If you've never customized your key bindings before, be careful about the key you choose to bind to these functions. You don't want to overwrite other crucial keybinds accidentally.
Once your Interact With Target key is set, you no longer need to worry about crowds or mammoths. Just /target who you want to interact with and hit the appropriate key. It will bring up the dialogue prompt, and you're set.
World of Warcraft: Cataclysm has destroyed Azeroth as we know it; nothing is the same! In WoW Insider's Guide to Cataclysm, you can find out everything you need to know about WoW's third expansion, from brand new races to revamped quests and zones. Visit our Cataclysm news category for the most recent posts having to do with the Cataclysm expansion.