I think my biggest piece of advice would be to just sit down and write. Seriously. Because I *know* how hard it is to do that, and yet, it really doesn't have to be. No one but you is ever going to see what you write as you write it, and it's okay if it's not perfect the first time through. You can always fix what you don't like later. Once it's down on paper (or computer, as the case may be) then you can improve the parts that you aren't happy with, but it's actually easier to do that when you have a large chunk of it written. And once you have something written, you can ask someone to look it over for you (this is, to me, one of the greatest things about LJ -- it is SO much easier to find betas than it used to be! I never had my work beta'd for years and years because I didn't really know anyone to ask).

But the big thing is just to write. The gorgeous thing about writing is that you really can't mess it up. It isn't performance art; anything you don't like can be fixed. :-)