How to use a 16x1 Multiplexer?
It would be helpful if you provide the part number of the multiplexer you are using.

However, a 16 input mux must have four digital "Select" or "Address" inputs - the bit pattern you apply to these inputs will select which one of the data inputs is fed to the data output.

The mux may also have one or more Enable pins which must be set correctly for the mux to function.

The datasheet for the part you are using should contain the necessary information to use the part correctly.