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 [b]leader[/b]. 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 [b]keep[/b] 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. [b]Encourage[/b] 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), 

[hider]
[code][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]
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[/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]
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[/list][/code]
[/hider] 

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.