You've written your magnum opus, the piece of interactive fiction you always wanted to create. Congratulations!
Now you distribute your game and tell people about it, that's what.
Granted, "distribute your game" can cover a lot of ground. How much work you put into doing so is up to you. Some of my games I've merely uploaded to the IF Archive and then announced their existence on rec.games.int-fiction. With others, I've gone all-out and made announcements in several places and uploads in several others.
But let's start with what should be your first step, regardless: upload your game to the IF Archive, the world's foremost repository of interactive fiction.
Head on over to the IF Archive's upload page. You'll need to enter your name, email address, describe what you're uploading, and select the file (or .zip archive of files) to upload. A word about the files you upload. If you're using a language which creates platform-independent files, such as TADS or Inform, consider also creating and uploading Windows- and Mac-specific versions of your games, especially if you're planning on adding your game to other software archives. If you mainly intend for your game to be played by more knowledgeable players, though, you don't need to bother.
Once you have uploaded your game, you're ready to announce its existence to the world. Do so on the Usenet newsgroup rec.games.int-fiction. It is usual to put "[Announce]" at the beginning of your message's subject line. Also mention that your game will shortly be available from the IF Archive in the /ifarchive/unprocessed/ directory, and what directory it will be moved to, if you know.
In your announcement, give some detail about your game. If you want, you can talk about the game's genre, or theme, or merely give some of the opening text. You want to get people interested in it, but don't over- or undersell it in doing so. "This is the best game ever!" is not likely to be taken well; neither is "This is my first game I hope it's okay and you like it."
As an example, here's the announcement I made for Common Ground, edited slightly.
Subject: [Announce] Common Ground
Great, just great. First you oversleep, end up being late for school. Then the whole day sucks so bad you can barely wait until seventh period's over and you're free. Now you've wasted so much time watching TV that you've gotta rush through your makeup or you won't be ready for tonight before April gets here with her wheels. Mississippi may say you're old enough to drive, but Frank sure doesn't agree.
Man, you can't wait for all this to be over.
Common Ground is a new TADS adventure from Stephen Granade. Currently it can be found in ftp://ftp.ifarchive.org/unprocessed/Ground.gam. In a short while it will move to the /if-archive/games/tads directory, though the filename will still be Ground.gam.
Common Ground. One night in a three-story house.