If you’re talking comics DC (and most publishers) don’t accept unsolicited submissions. Even the writers’ workshop Applejack mentions requires you to have published credits and almost all of the people selected had toiled for years in the indy trenches and produced some amazing stuff (full disclosure I worked on a project with one of them so I’m a bit biased).
Comics is really tough in general. Working for DC is like playing for the Yankees. It’s something very few people will ever get to do and the road to get there requires alot of money and effort.
To get your start in comics you’re going to need to produce something on your own. In order to do that you will either have to draw it yourself or pay an artist (if you get incredibly lucky you might find an artist willing to collaborate for nothing but that’s rare and generally speaking you get what you pay for in terms of quality). Comics are easy to write badly but difficult to write well. I suggest reading scripts (a search online with the writer’s name + script is a good start) but you should also start trying to read comics more purposefully; look at things like panel transitions and consider the amount of dialog in each balloon, how captions are used, the POV of each image etc.
A few books I definitely recommend reading:
The DC Comics Guide to Writing Comics by Denny O’Neal: probably the best book on writing comics since Denny O’Neal is not only a brilliant writer but a great editor and his book is short and to the point with lots of great advice on comics in general.
Understanding Comics by Scott McLoud: A comic that explains what comics are and how they do what they do.
Comics and Sequential Art by Will Eisner: This one is more practical for artists than writers but as a comic writer you need to understand art even if you can’t draw yourself.