Bitmap files are not Notepad compatible, and aren't just a block of simple data either.
Bitmap files are stored as binary data, not characters and are not intended to be human readable. When you open it with any text-type editor application, if you are lucky you will get a (pretty meaningless) hexadecimal display. If you are unlucky, you will get a display full of odd shaped blobs and question marks for the most part.
You can read a bitmap file directly into your program, as a sequence of bytes - but unless you process those bytes correctly, it still isn't going to make much sense; there is more in there than just the pixels. Wikipedia
] explains it pretty well.
There is an easier way: read the BMP file as an image and let existing code handle the file format: it will then present you with data and (normally, depending on the system) functions to access and manipulate the image data on a pixel basis if needs be.
I have no idea what language you are using, or system you are building for, so I can't help you any further - but start looking for an image class or function and start from there.