Send data from PIC microcontroller to esp8266

Since both PIC18f2550 and ESP8266 support Serial communication, the best way to go with this is to use the PIC to read the data from all the sensor like Temperature, humidity light etc and then send these values serially to the ESP8266.

The ESP8266 can then receive these values and publish the on a webpage with necessary graphics. In this method you need not use any AT commonads and is the easiest way. You can program the PIC using MPLAB and ESP8266 using Arduino IDE.

You can refer this tutorial but you have to replace the Arduino with PIC: https://circuitdigest.com/microcontroller-projects/iot-garbage-monitori…

Using AT commands

The 2nd method is what you are asking for. In this method you have program only the PIC MCU and the ESP8266 will again be connect through serial but instead of programming it separately you can control it through AT commands from PIC.

You can visit this tutorial https://circuitdigest.com/microcontroller-projects/interfacing-pic-micr…

for reference 

If I were you, I would have preferred a ESP32 in place of PIC and ESP8266 combined.

ESP32 has ADC, DAC and PWM peripherals and is much more faster than 8-bit PIC18f Architecture. Since there are many clone versions are ESP32 the price difference is also very less