Want to write? Start reading.

You have to read. A lot. And you have to read good books.

You should read a lot before you ever start writing. Reading is how you learn what a good sentence feels like. Reading helps you develop your command of the finer points of grammar and syntax. Reading improves your vocabulary. 

Reading is also the best way to learn about plot and structure. If you don't know where to begin and to end, you need to read, to learn the shape of a story. Movies can also help you with learning about narrative structure, since they compress the whole enterprise into about two hours. Watch, in particular, how they open and how they introduce characters.  The wedding scene in "The Godfather" and the montage introducing the gang in "Ocean's Eleven" are remarkably cool and efficient examples of how to introduce a large cast quickly, without resorting to excessive exposition.  If you want to learn how to establish characters, those movies are a good place to start.

"We have three minutes to introduce nine characters.  Are you in, or are you out?"

You can also learn a lot from watching TV, if you watch the good shows and pay attention to plotting and structure.  Television's narrative objective is to sustain narrative tension and viewer interest while moving the plot slowly. In a novel, things happen faster, and maintaining a status quo is less of an underlying objective, but tension remains important.  You can learn some good story tricks from serialized dramas like "Lost" and "Mad Men."

Reading will also help make things like dialog and description second nature.  Dialog that works on the page is a little different from the way people actually talk, but it feels natural. And many things people say, the pleasantries and small talk, are excluded on the page. If your dialog is clunky or awkward you need to read more, to learn how to write it.   Description should be evocative without getting purple or taking up too much space.  The best way to learn how to do these things is to see them done well.

If you aren't well-read, you are probably working with an incomplete toolbox.