We don't specialize in this sort of thing, but...
You can get the free Halo Custom Edition over
here on C|NET. If I'm not mistaken you need to have the commercial version of Halo for this to work.
You can get the Halo Editing Kit over
here at FileFront. The HEK includes tools like Sapien (for scenario editing), Guerilla (for map look-and-feel editing), Blitzkrieg (for exporting 3dsmax/Gmax maps), and Tool (for compiling maps).
You'll most likely also need a copy of 3ds max Studio.
This is not free software. There's a free version known as Gmax, though it was discontinued. I haven't gone looking for it, so I don't know if you can download it anymore.
For everything else I'm going to refer you to
HaloMods.com, which is a good starting point. That's where I started to leaern this sort of thing.
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