Layers for 3b hair?

Hello everyone! I have been wanting to cut my 3b hair into layers, but I have been nervous to do so. I am afraid that one layer will be cut too short, and I don't know if it would look good or not. I've gotten layers before, but just two tiny ones. My hair is half way (or a little more than half way) down my back. I have no idea how many layers to get or if I should get long or short layers.. I dont even know the difference between long and short layers. Can somebody help me?