First, I'd create a business model. Considering MM stands for massively multiplayer, you aren't talking about something that runs on your laptop over your local WiFi network, with the game going down every time you close your laptops lid. How massively do you want to get, how much hardware are you going to need, what's the bandwidth, Are you going to run it alone? If so, who does support when you're sleeping? If not, are you going to hire additional people, or depend on volunteers who may drop of the earth when you least expect it, etc. Once you know your initial requirements, how are you going to finance it all?
Second, determine what's the theme going to be. Vampires? Star Wars? Tolkien Fantasy? Contemporary? Superheros? Dungeon Crawl? Something else? What's the goal for the players? Hack-and-Slash? Treasure gathering? Puzzle solving? Role-playing? Exploring new worlds? Something else? How are the global game mechanics going to work? Is it going to be turn based? Time based (either real, or in-game)? Event driven?
Third. Make a list of all the software out there that's already been doing text based RPG, that have been developed over the past 20 years. Sit down in front of the mirror, and for each entry on the list, explain to yourself why reinventing the wheel is better than reusing existing software.
Fourth. If your plans are now just one sentence "create a text-based RPG", and then next thing you do is ask Perlmonks for meta-help (as you're asking for which software can I use to help me), you're going to need all the luck in the world.
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