well... you have 2 options.
I use ZED (or used to... as I am just too lazy to make anything these days I suppose... or too busy) which is provided with the Tribes Tools Pack, which you can get by following the link in my signature to my site and then to the downloads section. Included with the link in my signature is a sort of intro of common questions and problems people have with making it work, and how to fix them.
The other option... which might be better as there is much more "tech support" out for it, would be WorldCraft also known as Hammer... it may even have a new name these days, but I don't know. I cannot remember where to get it, but I am sure if you type in "World Craft 3 Download" in to the google, yoou will find results.
Although I would be able to help you should you get into the ZED instead of World Craft, I reccomend World Craft, as members like Hersh still actively use it and would be best suited to help you should you need help.
It's up to you... but you can always follow my signature link for a start on the tools pack... you will need them if you're going to make your own textures anyway (the pack contains the custom pallete files you'll need) among many other usefull tribes-exclusive tools.
Peace and good luck!
