I have scoured two Nikon cameras, d3 100 and d3200, but neither of them can focus automatically or take automatic shots. How can I set it without a switch on my head? What a headache!

If it can be determined that there is nothing wrong with the hardware and software, and there is no focus mode switch on the lens, then there should be a problem with the focus mode setting of the fuselage. You can first see if the focusing mode displayed on the camera screen is MF, that is, manual focusing mode. If so, you need to switch the focus mode. Because the machines D3 100 and D3200 belong to Nikon's simplified entry fuselage, the control design is relatively poor, and there is no focus mode switch button or direct switching on the machine. You can do this: press the I key on one side of the screen (Nikon officially calls it the information editing key), then use multi-selection, press the keyboard with up, down, left and right keys on the other side of the screen to select the focus mode, that is, MF, and press ok to enter the options menu and select the required focus mode. If you don't know which mode to use, use AF-A first, and then press ok to change. If you have to specify the operation settings of these two cameras, you have to look at the actual product manual. Nikon officially provides an electronic version, so you can find it yourself.