I'm a newbie photographer?

I'm looking for (affordable) lenses for my Nikon D3100. I've had my camera for a year, and have yet to buy a new lens. I'm totally lost in what to buy and would really appreciate some feedback. I've been looking at the Tamron 75-300mm F/4-5.6 LD AF-D For Nikon, but after more research saw that it wouldn't auto-focus with my camera. Any suggestions?

Get the Nikon 70-300 VR AF-S lens. It's not top pro glass, but pretty darn good.

Just look over the thumbnail images to get an idea how useful it is. Click on any if you want to check out the image quality.

Http://www.kenrockwell.com/nikon/compatibility-lens.htm#dslr

Yes, your camera would only AF with AF-S and AF-I Nikon/Nikkor lenses. Check the equivalent with other lens brands.

http://keerok-photography.blogspot.com/2011/05/lenses-so-many-of-them-there-is-no-best.html

Buy a fast prime lens, best thing for learning on, zooms make you lazy: P
Perhaps the Nikon 50mm f1.8G?

It really does depend on what you like taking pictures of though. If it's portraits, then ideally you want something around 70mm (for your camera anyway). If you like taking landscapes then something below 18mm is good. For sports or animals you need something longer, around 200mm and/or more.