A lot of fine folk already covered a lot of what I would say, but I'll still put down my usual receipe.

-You as a GM are responsible for keeping your players invested by keeping the game moving. Do not let it get hung up waiting for one or two people. Set a loose time frame, or keep an eye on momentum, and when it sags, keep things trucking along or you will have people start to leave due to inactivity.

-First few months/ story arcs are essential to get through to establish your core player groups who feel the game is going to last, as well as people start to build relationships with one another and as a GM. People will also be attached to the characters after interacting with them for a while, so GMing starts to become more routine maintence verses trying to actively keep people invested.

-Take time to format your interest check and OOC. Colour headers/ find a banner generator, use graphics, and keep each section distinct. Generally I'll lay it out like;

* Title + in-character exposition to hook players/ hint at the theme and core conflict of the game

* Lore and setting summary, like you're reading the back cover or inside sleeve of a novel. No OOC bits here, just straight up "This is fantasyland, where nobody gets along. A long lost princess named Rabies is back with never before witnessed magic. Fantasyland is falling to Rabies, but a group of heroes gather in a tavern to play their next move to stop Rabies. This is their story." Kind of thing. It tells players exactly what the setting is, core conflict, and who the characters are.

* OOC stuff where you explain things in a more clear and detailed way, like the posting standards, what the game is about, a bit of the lore that's essential to get started, your rules and what not, and so on so forth. Ask yourself if there's been times you've wanted information going into an RP; add that here.


* I usually do a lore codex here. It's pretty optional

* Character sheet skeleton

- Do not be afraid of having pretty elaborate rules. If you outline exactly what you're looking for players to follow, you cut down on immense levels of conflict and people power gaming. I even go so far as to put equipment restrictions. The hard part about things like fandoms is people will try to treat it like a video game if you don't stop them ahead of time.

- Do not be afraid to decline players who aren't up to your standards. I like to give feedback on character submissions part way through the application period, that way people have a chance to edit and get it up to snuff. If a character doesn't fit the setting (e.g. a librarian in a mercenary company), don't be afraid to say so. Offer ideas how to alter key points and keep things meshing well. People are often open to criticism, especially if you help them make changes. 

- Similar note; You can reject and kick players for any reason. If someone rolls up with a stellar sheet but you know they have a bad additude and cause issues in other RPs, you are not obligated to take them.

- Set yourself a reasonable player cap. I've been GMing for nearly 6 years and I still don't like to handle more than 8-10 players.

- Keep the OOC somewhat active. Assume not everyone uses IM services like discord and post all meaningful notices and information there. Stuff like mini writing exercises are great!

- This is a big one: plan ahead. Even before launching the game, write down a bunch of world building info like NPCs, locations, history, do a plot roadmap, write down potential plot points and conflicts, everything you will need to firmly establish the game out of the gate. Think of it like a starter pack in a game and further plot points are expansion packs. When you do your interest check, have the OOC already finished so people aren't waiting and have the intro well under way. The less people have to wait on you, the better chances you have of success.

That should be a pretty good start for you, best of luck!