What is "that"? Google returned an overwhelming number of hits when searching for "that".
If you read the whole topic you will know what "that" is.
There are various approaches open to you, but the simplest one is to read the file character by character. If the character is not a comma, append it to a char array buffer. If it is a comma, parse the content of the buffer to a number and clear the buffer. This will give you a sequence of numbers read from the file. I assume you know what to do with the numbers from there on.
My difficulty is how to read character by character, after it is all the values are printed on the serial monitor I don't know how to read them.