Hi,
SD card can be interfaced to microcontroller using SPI communication & 8051 does not have built in SPI function to interface with SD Card, you need to create SPI function, which is little difficult & time consuming. Suggest you to switch to latest versions of 8051 based controllers which has built in SPI function & it would be easy for you to interface with SDcard.
Basic Interface of SPI includes four pins –> MISO, MOSI, SCK & SS
ThanQ