Honestly the best advice I can give is to NOT be afraid to use published missions/adventures while you are getting your feet wet as a GM. That way all the structure and information is right there for you. I have been gaming a long time and when I start running a new system I always prefer to at least read through, if not run a couple of "store bought" missions/adventures just to get a good feel for the system as intended by the developers. Personally I find it an invaluable experience. That way plot etc. is handled while I can concentrate on character interaction
This is the most universally useful advice. Without knowing anything about you or your style, I can fully endorse using a couple of the published adventures as a stepping stone toward developing your own sense of style.
Other than that, I'd say, try to pay attention to setting. If you can picture a scene in your head, that's the first step towards placing the characters in that scene. When you're designing encounters, try to come up with a character to be the avatar of that encounter. There should be one real face in any mob of faceless adversaries. He will be your mouthpiece and your means to humanize potential foes, offering your players other avenues than "kill everything that moves" in order to resolve a dispute.
Players will draw weapons on faceless ork gangers every time, but if you've got a grizzled ork named Charley with shock-white hair and a burn scar down the side of his face, your players might chat him up before they shoot him like a bunch of psychopaths.
