How to I make the Tx and Rx pins on an ESP-8266-01 into GPIO pins?
Add this to your code at the beginning of set void setup():

//********** CHANGE PIN FUNCTION  TO GPIO **********
//GPIO 1 (TX) swap the pin to a GPIO.
pinMode(1, FUNCTION_3); 
//GPIO 3 (RX) swap the pin to a GPIO.
pinMode(3, FUNCTION_3); 
//**************************************************


You will no longer be able to use the Serial Monitor as TX will now be a GPIO pin and not transmit Serial data. You can still Flash your device as when you boot the device in flash mode it converts GPIO1 and GPIO3 back to TX/RX. Once you reboot into regular running mode GPIO1 and GPIO3 will go back to being GPIO pins. 

To change GPIO1 and GPIO3 back to being TX/RX for regular Serial Monitor use add this to your code at the beginning of set void setup():

//********** CHANGE PIN FUNCTION  TO TX/RX **********
//GPIO 1 (TX) swap the pin to a TX.
pinMode(1, FUNCTION_0); 
//GPIO 3 (RX) swap the pin to a RX.
pinMode(3, FUNCTION_0); 
//***************************************************


If I understand the ESP-01 correctly the pins CANNOT be both TX/RX and GPIO pins at the same time.