I'm not sure what you're trying to do, but is it: read a character from serial and then keep sending the analog reading it selected?
If so, you need to shorten the first if statement so that all it does is check for serial.available and read a character if one is there.
All the rest of your code needs to run every time loop runs. As you have it, all your code is controlled by serial.avaialble. If there's no serial input, nothing else happens.