Like writing a book, the only way to get good at it is to acknowledge that your first is going to suck, but to write it (run it) anyway. It's "like jumping off a cliff and building your wings on the way down," as hbomberguy puts it. The only way to learn——to truly learn——is to do. No amount of advice you receive here will ever even remotely prepare you for the real thing.

[hider=picture big :(][img]https://media.licdn.com/dms/image/v2/D5622AQHdmXpeosBLeA/feedshare-shrink_2048_1536/feedshare-shrink_2048_1536/0/1730818842570?e=2147483647&v=beta&t=ro6ucg6QY2PUJC74Xh_wUq2cu6OVBi9BSJaJnTwAuOc[/img][/hider]
That said, a few tidbits of woefully inadequate, disjointed, but not entirely unsound advice which come to mind:
[list][*]Per Warren Spector ([i]Deus Ex[/i]): "Players do the cool stuff. NPCs watch players do the cool stuff." Don't give any epic, exciting moment to yourself which could go to a player instead. DMPCs are——without a [i]very[/i] deft hand——a trap.
[color=2e2c2c].[/color][*]Sandbox stories [i]seem[/i] like the best of both worlds: you lessen your own workload while also offering the players greater control over the shape of the plot. But this only works with a confident group, willing to take risks and seize their own agency...a familiar and trusting friend group, in other words. Whereas in my experience, roleplayers in general, but especially P.U.G.s, would rather wait for others to post first to see what happens, test the waters, and make absolutely certain they won't upset anyone or disrupt anything; they gripe that they "don't know what to post"; they second-guess, hesitate, and generally fret about ruining someone else's plans. Mount this atop the general stresses of writing, editing, and proofreading The Perfect Post and it leads to collapse more oft than not. Sandbox games are a first-time GMing death sentence. Please just create a real plot.
[color=2e2c2c].[/color][*]Constraint breeds creativity. A "you can be anyone and do anything" design ethos sounds nice, but results in characters built to be baseline-competent at most things, useful in most situations, pertinent to most plots; thus, milquetoast. Players sacrifice some real texture in their writing when they're tasked with making a character who won't be left behind or rendered redundant or outright useless no matter the direction in which the story travels.
[color=2e2c2c].[/color][*]If you're unsure about an application, the sooner you address it the better. Know when to say "no" to someone who didn't understand the assignment. If (say) you know you want a dark, sinister tone for a setting filled with very real threats, being willing to say "no" to the lovable rogue joke character is absolutely critical. Letting that character into the cast will only lead to resentment when he seems like he's purposely "sabotaging" the tone/mood/atmosphere you're aiming for. Boundaries are good. Knowing what your game is, and what it isn't, and communicating that sooner than late to filter out the bad fits, is even better.
[color=2e2c2c].[/color][*]Have a target audience narrower than "literally all rpers on RPGuild." You [i]want[/i] to alienate [i]some[/i] people. The product that tries to appeal to everyone is inoffensive but ultimately flavorless.
[color=2e2c2c].[/color][*]Nobody wants to do homework, study, and take an entrance exam to join an RP. Save the epic lore for [i]optional[/i] OOC hiders, or better yet, sprinkle it into your posts through tasteful, naturalistic exposition.
[color=2e2c2c].[/color][*]If you don't already, start reading. Read, read, read. And watch, and play, and read some more. The wider your literary/artistic exposure the more original and unique your own stories will turn out.
[color=2e2c2c].[/color][*]Don't trust anyone who says they've been "craving" what your thread is offering. Cravings are fleeting and momentary. Cravings are outgrown and lead to ghosting.
[color=2e2c2c].[/color][*]You're allowed to copy Matt Mercer until you discover your own style.
[color=2e2c2c].[/color][*]If you have stage fright IRL, GMing triggers the exact same neural pathways. It's terrifying putting yourself out there like that, especially for the first time. It's also electrifying, validating, vindicating, and utterly addictive.
[color=2e2c2c].[/color][*]If it turns out you're pretty good at this whole GMing thing, expect to become the Forever GM in your friend group when no one else wants to step up. There is no escape from this. Welcome to hell.[/list]