In my jpeg file there are few FFDA markers. From which marker my data starts, and how do I know from which marker I decode the file?
Your JPEG is probably progressive which means you have to decode the data after at least the first FFDA marker, which will bring you an intermediate result.
If this is your first attempt at making a JPEG decoder you should choose another image and try to implement a baseline decoder instead. Progressive images adds a lot of complexity to the problem.