There's a lot of good advice out there for writers, but the two best pieces I've heard (and will always pass along) boil down to:
1. Read. A lot. Of everything. It's grist for the mill, sparking ways to do things differently or the same, as well as providing ideas.
2. Finish. You can't spot what needs fixing let alone get something to a reader until it's done... and completion really does give you confidence to do more. Don't let a shiny new idea steal you away. Finish what you start!