How can build a desktop app to detect steganography image? What are the things I should focus on considering my basic knowledge of programming? Need some guides

Quote:

I have actually tried to find previous work but all are already made apps

You may have to build your own solution.

Quote:

How can build a desktop app to detect steganography image?

The general principle of steganography is pretty well known, the problem you will encounter is that details are left to programmer.
- The way data is embedded in picture depend on taste and skills of programmer, there are tens of ways to do it.
- Since data can be compressed and encoded, you can't rely on readability of extracted data to identify that what you read is actual data.
- Some garbage data can be appended at the end of actual data to prevent too easy detection of boundaries.

So steganography detection without knowledge of actual method used for encoding and embedding can be pretty tough.
We do not do your HomeWork.