There are a gazillion cameras out there that are fine starter cameras. Many cameras are better than the D3100. But b/c it doesn't have an AF motor in the body, it's a bit smaller and weighs less than most DSLR bodies. Which means that--for a new shooter--you're more likely to carry it with you. And the BEST camera is the camera you have with you...not the one back in your house or your hotel room. So the D3100 is a solid choice for a beginner.
The best advice I can offer has nothing to do with what camera to buy. B/c the truth is: equipment is overrated--especially for a beginner.
1. Read the manual. Then read it again. Then buy a book (not produced by Nikon) about your camera body and read it. Your camera (even if it's a D3100) has far more capability than you'll ever use. So learn about some of it--maybe 20%. No, I'm not being insulting. Modern DSLRs are technological wonders and have more stuff than the vast majority of photographers need or use.
2. Look at how you learn best. Is it by reading? Viewing? Person-to-person instruction (like a class or coaching)? And once you have that clear, start getting some instruction using that mode of learning (books, websites, classes/seminars, a coach, videos--whatever). Bryan Peterson is a great person to learn photography from...has a lot of short videos up at the
Adorama website and has authored a ton of books on a range of photography subjects and also does workshops. And if you buy one of his books and love it, then you'll know he's got about 10 other books and about 100
Adorama videos you can tap in to. What you want to learn is not so much about your camera (reading the manual helped there) but about the art of creating a photograph. Not capturing one, but creating one. Peterson will help you with that. Michael Freeman is another good option to look at.
3. Go to exhibits and get awed. And learn.
4. Give yourself a weekly challenge. For instance, find natural "S" curves around you. Take 30 different perspectives of a household object (a pencil or scissors or a glass) without using photoshop. Use negative space. Shoot high key. Shoot low key. Use DoF. Blur movement to capture a sense of motion in a photograph. Shoot drops of water. Shoot smoke. Look for patterns in nature. Shoot some street photography and focus on emotional connections. Try shooting an new born or infant. Give yourself a challenge like this every week for a year. And keep a journal/notebook to jot down comments about what you learned, what questions you have, what progress you think you made on that assignment.
5. Don't get fixated on the technology. You're a beginner. You're going to produce some crap photos even if we put a $10,000 camera with pro lens (that cost more than the camera) in your hands. Technology only starts to make a difference in photography once you have enough skill to be able to manipulate it--to now how to utilize ISO 64000, to know how to rear curtain sync an off-camera speed light, to know how to do light painting, or set custom white balance to pull out the red in the autumn foliage or shoot pro basketball players by the side of the court during the game. With a D3100, heck, with a decade old D40, you have the tools to learn photography and create good photos. So don't go out there and try to find the "best camera" b/c right now, the best camera for you is one you will carry and use and learn with.
Enjoy the addiction, er...I mean journey.
Ed