Well to start with, I have to assume you're readin in bytes of the file.
A byte is a number from -128 to 127, bytes in files tend to be unsigned, so 0 to 255. I imagine these are the numbers you're seeing.
You then have to convert these bytes, by using the format specification, into something the program or you can understand.
WaltP was correct in saying that you have to look up the file format. Otherwise, the data from the file is just nonsense ^^