What advice do you have for someone who wants to GM. This can be anything from the actual writing/world-building/other story-related topics, Overall presentation of the RP, to creating/using mechanics in a tabletop Rpg style thread. Throw it all at me.
I've been GMing for a few years. I'm much more of a novelist and a story teller, but I've learned a bit about world building and various rules. First thing is first. The best you can do for your RP is to be a good leader. Not boss. But leader. Be courteous but firm to those who have shown interest in your RP, or else you'll find yourself without players (unless they're already part of some friend group you're in, but let's not get into that). Make sure you lead them and guide them and bring them interest into your world. Don't think "I'll let them play around a bit."
You need a subject you're willing to be passionate about, and enthusiastic for. You need to be patient for players, but also ready to go and keep going. If you keep making RP's and then dropping them, people will notice and stop joining.
Interest checks and OOC title posts should be well formed, perhaps with a cool picture to grab player's attention, but also needs to be neat and precise, with an overall description of the RP you're making, and perhaps extra notes or various rules, or elaborating on how you plan on running it. Make sure to provide all the info you can possibly give, and always be ready for questions. Encourage them. Being a GM is a learning process just like being an RP'er.
Make sure when you make a character sheet (and you can find examples of them in many RP's. Lemme grab one for you from my own),
[B]Username:[/B]
[b]Character Name:[/b]State your name. (Put your alias or title in parenthesis)
[b]Race/Species:[/b] I'd prefer human or half human, but PM me if you wish for something different.
[b]Gender:[/b]
[b]Age:[/b]
[b]Career/Class:[/b]
[b]Weapons:[/b] [list] [*] [*] [*] [/list]
[b]Physical Description (as detailed as possible please, pictures not accepted.):[/b]
[b]Armor:[/b]
[b]Equipment/Other[/b]
[b]Mental Description/Personality:[/b]
[b]Goals:[/b] Why are you in the Southland? Fame? Riches? Adventure? Fleeing the North or come to settle an Old Debt? Perhaps you live here already within the city, or were born here?
[b]Background/History:[/b]I realize that I don't have a wiki for my info, so I will allow bullet points for a history. If you want to use paragraph form however, feel free! [list] [*] [*] [*] [*] [*] [*] [*] [/list]
Make sure you put it in a [code] [/ code] tag, so bolded letters and underlined stuff can easily be copy/pasted onto their reply screen and they can just begin filling their sheets out.
Also, it's also good to make the same character sheet again without the code tag, and with each subject, write what you're expecting from dedicated players. Like, for personality, elaborate you want at least 2 paragraphs and explain yada yada stuff to you, and so forth.
Be punctual and on time for stuff. Don't go back on your word. I plan on making a world building guide for medieval settings at some point, but I'll give you the same advice I'll give players who want to incorporate magic in their world.
Ultimately how you want to do things is up to you, but if your rules are loose, at least try not to contradict yourself.
Take what time you need to plan things out. Waiting a week or a month extra with launch is worth it if it means the RP will survive longer.
Oh, and get a good co-GM or two to help you plot things, or at least someone to discuss (or argue) with. That'll help find flaws you yourself won't easily spot.
Don't be afraid to start an RP over. Sometimes, flaws that snuck in during planning cannot be solved any other way. Its better for all to start it over as a new version than to keep going with something broken.
Never assume people will stay, because there's not a single successful group RP that didn't lose players along the way. Always leave space, both in OOC administration and IC plot, for replacements. Some people may close applications at a threshold, but I leave it open even when the group is stuffed (new guys go into queue at that point), as that will keep a steady flow of replacements coming.
Don't even assume yourself will be dedicated all the time. Like Ellri said, a good co-GM can back you up when writing gets difficult for you. Depending on how much you trust your co-GM, you may also wish to tell them future plans for the RP. When someone else has the roadmap, they can continue the game even if the original GM is absent. Long time RPs tend to become community efforts.
I'm against having pre-assigned roles or characters occupying non-replaceable roles. Just one of these pre-assigned or pivotal players leaving can cripple the entire story. Instead, start people off as somewhat generic and define them as they go along.
Most importantly, don't be afraid to make mistakes. Consulting your co-GM, players and even potential candidates (in interest check), to smooth out your plans from different perspectives. You may restart or reboot an unsuccessful RP, while doing so, look back to see what went right and wrong, and improve based on these lessons.
A few things I wish someone told me when I decided to start doing the GM thing:
Do not be too ridged with your story. Roleplayers can be extremely unpredictable. If too many things "have" to happen in your story, especially as far as character interation goes, you will be sunk. This is especially true for the beginning when you don't know how all the characters are going to act.
Have flexible plot points that don't require everyone to be there. If you can move the plot by yourself if the need arises, all the better. Though better still is if you can have events happen around the other players. Most players like the idea that their characters are important in your world. Though there are some that like alone time to develop their characters too.
It doesn't hurt to occasionally ask how a player is doing. It's best to get an update regularly instead of just when you think something is wrong. I tend to ask all my players how they are enjoying things about once a month. Do this via Pm. You might be surprised with what they say.
Take input from everyone. You don't have to take everyone's suggestions, but if someone feels like they're being ignored they won't stick around.
You won't become an epic GM in a day. Learn and grow.