A UART is a specific device within most microcontrollers, you need to consult the data sheet to see how to use it.
If you want to use any other pin for serial communication you have to write your own software to do it, we call it a 'software uart' or a 'bit-banged uart'. Your program is responsible for performing all the timing and shifting between serial and parallel streams. How to do it depends on which MCU you are using and to some extent what other things it also has to do. Software UARTs require precise timing so it often isn't feasible to use one if other demands are made on the MCUs resources.
Brian.