The only modding program that was designed for Halo that works with the trial version is HMT v3.5 (Halo Map Tools) by MonoxideC.  I recommend you download it and then mess around with it a bit, learn the interface some.  The other way to mod Halo Trial is by using a hex editor.  Hex editing takes a lot more time to edit, and a lot more time to learn.  If you're still interested, I recommend Hex Workshop by BreakPoint Software.  You'll also need the hex maps (lists off all the offset's locations, etc) for the trial version.  The hex maps were made by Iron_Forge.

    To get the most features out of HMT you'll need a good set of plugins.  I recommend this set, it's Phenomena's final plugin pack.  All of the plugins were made by Phenomena, except weather partical and damage, which was made by Plaz.  You might want to backup your original plugins before copying them over with these, as I like to switch back and forth between original (basic) and the new plugins (advanced).

    This page is still under construction.  If you have any questions, feel free to stop by our new forums and ask.