Why can't i take pictures in viewfinder?

I just got a nikon d3100 dslr and this is my first dslr. I can take pictures in live view, but not in viewfinder unless I switch it from A to M on my lens. How do I take pictures while in viewfinder while in A

Turn off liveview

Read the manual.

In addition to reading and studying the Owner's Manual for your camera, watch this video tutorial:
http://www.nikondigitutor.com/eng/d3100/index.html Who better than Nikon to teach you all about your Nikon D3100?