Read copiously. Read copiously within the genre you want to write in and copiously outside that genre. If you plan on writing fiction, don't forget to read nonfiction - this can be a double blessing, because many times a fiction writer needs to write about something they don't have a lot of background in. Thankfully there are books on almost every subject you could think of.
A pet peeve of mine is writers who aren't familiar with weapons who then write about them. Really the hallmark of a lazy writer, there really is no excuse! This goes for journalists too. It's not hard to find authoritative texts on practically any subject you could think of.
So, in short, read to learn good writing habits and to do your research!