This is a pretty bog-standard facility available in many of the "canned" developed flavors of TW - every on that I've check out but the core original.
MPTW's a very popular one, with quite extensive handling of "tiddlers as tags" (in fact I believe Simon was the one who really fleshed out many of the "taggly" concepts that are now part of the newer versions of core TW)
http://mptw.tiddlyspot.com/The specific functionality you're looking for, if you're rolling your own from core and don't want the other bell's and whistles, is here:
http://mptw.tiddlyspot.com/#NewHerePluginBut on the other hand, if you really like bells and whistles, my personal favorite, but perhaps a bit complex for someone starting out;
http://twt-treeview-executive.tiddlyspot.com/Hope this helps