You definitely need to give people enough information to know what they're in for.

Outline the setting, do some world building of sorts and answer the "who, what, where, when, why" of what the game's about, including what the player characters are supposed to be, as well as outlining generally what kind of conflict is going on, or at least what the story is going to be about. Treat it kind of like the summary of a novel, just with more game-oriented direction. Coming up with a hook should help.

You should also state what your standards are, so like casual, 2 paragraphs, whatever. Also keep in mind that zombies/ most apocalyptic RPs are pretty over-represented, so you will have to really try and sell people on why your game is different than a lot of the other ones, and having a clear goal can help a lot with that. If an RP is just about aimless survival, you probably aren't going to have a lot of retained interest; you need to have goals for people to work towards. Maybe it's something like trying to get to the coast, or a rumour of a safe haven, or even a way to stop the infection. It doesn't have to be anything substantial, but it does need to be there, otherwise you will end up with a few rounds of posts without any real progression of a story.