Nice photos. "Auto Mode" on the Nikon dSLRs is generally pretty good, as you can see here. If you want to get a book to better understand how to "take the training wheels" off of your camera and go Manual, this is the book I've read, and it's really pretty outstanding. Also, after you read it, go back and read your camera's manual---it will make a LOT more sense.
Link to Amazon
And, better glass always helps. The three Nikon lenses I'd recommend to start with are the
Nikon 18-55mm , the Nikon
55-200mm , and the Nikon
50mm normal. This is a relatively inexpensive set of lenses that should cover everything you'd need as a beginner. Alternatively, you could buy the Nikon 18-200mm to replace the first two mentioned lenses, but it's expensive ($800+), and nearly impossible to find.