It's important to have a decent editor to edit UnrealScript, that
understands the language.
While it's possible to write code in plain old notepad, it will be more difficult that way.
Some of the important features a good editor will give you are:
There are many possible choices out there. I personally use
While the base program is a step up from regular Notepad, it is important to install the unrealscript-specific addons to get proper syntax highlighting. An addon for this, along with instructions on use, can be found at http://romerounrealscript.blogspot.com/2011/10/setting-up-notepad-for-unrealscript.html
Happily, a large part of the UDK engine code is free for all to use and look at. Unfortunately, it is so large, that documentation for it is a little lacking. Instead, there is somewhat of an attitude of "Use the Source, Luke!". As such, you need to be able to do so as efficiently and easily as possible.
The UnCodeX tool lets you easily search for code in the existing UDK provided code I would personally say that trying to learn UDK without it, is not worth attempting.
I'm usually extremely paraniod about downloading "random programs I've never heard of", but seeing that it is developed on sourceforge, made me more comfortable, alongside the fact that it is frequently referenced on the UDK developers' forums.