[TheThan]

A good place to start is with pregenerated campaigns. That’s right, those boxes you buy at the gaming sore. Because it’s a premade campaign, you don’t need to sit down and write out a big long planned out campaign. That part’s been done for you. So all you have to do is study up the material and run with it. it’ll also give you a chance to practice adlibbing, as the players will more than likely do something the book didn’t expect. But that’s general player behavior anyway so get used to it.
Another good thing about the pregenerated campaigns is that they give you a bunch of maps and other materials. You know, the sort of resources that Dms are constantly looking for. I think most of us have a library of “Dming materials” we call upon to use when we Dm. the premade adventure will give you a good place to start when you start collecting your own little library.
Lastly it’ll also give you a formula to follow when you get around to writing your own campaigns. Naturally everyone has their own methods of doing just that. But at the very least, you can look at the premade(s) and get an idea of how the creators built their campaigns and you can use that as a rough framework for your own endeavors.

Many of us started out with nothing more than the DMG to go by (myself included). It's got some decent information in it but it does lead a lot towards trial and error. The premade campaign is a useful tool that you can add to your arsenal, that should reduce the amount of trial and error you will have to do. I hesitate to call it a shortcut, but it is definitely a useful tool.