I'm pretty sure there's already a fairly large Halo mod going on, but I don't know what happened to it. How much do you know already? You're not going to get very far into a project like this without experience. I would suggest starting out with just minor changes you'd like to see in the game. .ALO format is currently only supported by 3D max, which is not free and there is currently no workaround whatsoever. If you want to model, you'll have to pay. But if you model really really well in a free program, like Sketchup, Blender, Wings, or something like that, and you know a nice professional modeler, he might be able to convert the models over to .ALO.