Books, books, books! Buy/borrow them, read them, love them! Here's a list of books related to comic writing/creating specifically that you should get your hands on.
1.) Understanding Comics/Making Comics by Scott McCloud
2.) Word for Pictures by Brian Michael Bendis
3.) Comics and Sequential Art/Graphic Storytelling and Visual Narrative by Will Eisner
4.) Come In Alone by Warren Ellis
5.) Writers on Comics Scriptwriting Vol. 1 and 2 by Mark Salisbury, Andrew Kardon, and Tom Root.
6.) Panel One by Kurt Busiek and Neil Gaiman