First off, welcome to TreoCentral and the world of Windows Mobile.
Second, I highly recommend skimming through the user manual that came with your Treo. That will help familiarize you with the device, OS, and how they work together as well as where things can generally be found in the OS. I'm not saying you should study and memorize, just familiarize yourself.
Once you know where you stand and get an idea of what it can do, you'll have a better idea of what you actually want to customize and tweak.
As for what programs you need "to be able to make use of the add-ons and tweaks listed in this forum", that's kind of a loaded question. This forum covers a HUGE amount of ground. Some of the hacks and tweaks are simple registry changes, some are scripts (which would require MortScript), and some are whole programs themselves.
You'll have to narrow it down to something more specific, like "how do I make my Treo do _X_". If we know what it is you're trying to accomplish, I'm sure someone here can help steer you in the right direction.