You’ve learned the 89c51 mcu which is really great, as it can be viewed as a basic of all mcu. Now you better select some pic mcu like 16f877 and learn its manual/datasheet in detail. Ater you gain a fair knowlwdge about the hardware mostly registers, try mplab like IDEs to program the mcu.
Search for the schematic of any basic pic development kit and find out the programming circuitary part, try to implement the same.