I am a new altera fpga learner

well. that is tough Question.

because, it is depends on your environment.

first of all. SD card is basically connected with SPI wires.

SPI wires to where is you Qsys.

if your Qsys has HPS(ARM CPU) and OS runs on the CPU.

device driver on the OS manages SPI, you can just mount SD card.

if your Qsys just has NiosII.

you have to have some IP core ( translating Fat file system)

or run OS on NiosII.

you can even access to SD card directory.

I can not explain how to do that here.

you need to read specification.

that is tough and you can not read SD card by PC unless you write Fat file system by yourself.