As a GM, having a PC of your own is useful because you can use them to tell players information. But take care not to have them hog the spotlight. Also, try to only say "no" if something a player is doing would cause the RP to be less fun for others. Also also, rember that your players are just as good at writing as you, and they outnumber you. If you want a serious story and they want a goofy one, then you should have a goofy one.
It's good to have some idea of what's going to happen. Like you know that the bad guy is going to try and steal something at some point, and that he's going to kill so-and-so. And you need to let players know about these things, and give them chances to intervene. But the most important thing is you need to realize that it's really the player's story. So you need to read what players want out of it, and mold your story around them.
It takes practice to read your players, but you can try asking questions. Also reading sign-ups is a good source of information. Like in one RP I'm running, one of the players has ghost powers. So right now they're battling ghosts in a haunted museum. And another player is a vampire, and I'm probably going to introduce the dude who bit him as a major villain.