@JFK Being good at the whole writing thing helps, but it’s not the most important bit. There are plenty of RPs that have hooked me despite the GM not being the best writer, just as I am seldom the best writer in any of my successful RPs. You’ll wear many hats as a GM, but your most important attributes will be your “charisma” and ability to adapt. The better you are at handling people and navigating setbacks, the better.
Reputation is an important thing to work on. Participating in other RPs and being a swell player improves your reputation. Players will be more likely to sign up for an RP if they know the GM has a reputation for sticking it out or has a personality/style they like. A GM that frequently flakes out of other projects will be remembered by future players, and possibly avoided. If you join an RP, try to stick with it as long as you can. If it’s not for you or something comes up, let everyone know in that group. There will be no hard feelings that way (probably), and when you host your own RP later, players will remember your courtesy. As a new joiner of the forum, some may see you as a flight risk. But this does not take long to change. Inversely, do not become so focused on your reputation that you stick around players or projects that you don’t like. Reputation has some variance, and different people will look at you in different lights. One group’s “writer with fantastic prose” is another group's “Writer with purple prose.” The basic idea is to not be a jerk and bring your best.
You mustn’t be afraid to talk to the players. The glue that holds an RP group together isn’t just the IC but the OOC as well. Don’t be afraid to comment on posts. People like to know their posts are being read, and even a single sentence response can mean the world to them. It doesn’t have to be a complement, in fact, it would be better if you just focused on the content of the post itself. Wonder out loud if two characters are going to meet up, or comment on how underhanded one character’s dirty trick was.
Try not to rely on any one player too much. Regardless of how much you like their character or feel they are reliable, you should always have a plan B if the unthinkable happens. I have seen RPs grind to a halt and die because the GM thought they would have their favorite players around forever. Life can be unexpected sometimes, but the show must go on. It’s one thing to have a side plot that involves one character drying up, but the RP itself should never totally rely on one player. As the GM, you are allowed to take control of absent characters.
Don’t take things too seriously. Nobody else is. You may love the RP and put a lot of sweat into getting the systems and plot just right, but it’s rare your efforts will be fully appreciated. Your players care more about how you make them feel than how good your systems or plot twists are.
On the topic of systems, be aware of what you’re implementing. The more rules you add to a game the less literary that game becomes. The ronin on a revenge quest will pawn his fallen master’s sword for the new one with +2 DMG. Players become less interested in the story and more interested in acquiring power and making numbers grow.
Attention wanes quickly in this hobby. If you’re fortunate enough to get the interest of ten players, expect eight to make characters and six to participate in the RP, and only four to make it past the two month mark. Assuming you do everything right. Keep things moving. If your RP has a weekly posting rotation and one of the players has some setbacks, don’t keep your other players waiting. Everyone should know the rules. Find a way to keep the struggling player in things while keeping the post rate up. But play it by ear. I once had a group of college students that were all taking tests at the same time and called for a multi-month hiatus so that they could finish their studies and ace their test. They were all appreciative of the wait and all of them came back to resume posting at the required pace. But I’ve been doing this for a long time, and I’ve only done that once. Again, talk to your players. You’ll get a feel for what everyone wants and needs.
There’s more I could say, but I think that’s good for some “quick tips.”
If you want some more Gming advice, I strongly recommend
Dervish's Guide to GMing. It’s something I like to recommend to every aspiring GM and has loads of great advice. Doing is really where you learn though.