I am using x64dbg to explore image files on Windows. After the computer finishes prowling ntdll.dll it jumps to OptionalHeader.AddressOfEntryPoint. But this is not my main()-function.
The code that runs before your main function is the C Runtime (CRT) initialization code. There are ways to remove it, such as via the /NODEFAULTLIB and /ENTRY command-line options to the linker, but be careful what you wish for. If any of your code calls functions in the C standard library (such as printf, malloc, etc.), you will not be able to link your code into a final binary unless you provide your own implementations for those functions. Your implementations must be from scratch; you won't be able to rely upon any standard library functions, or implement them using any third-party library that itself relies upon the standard library. This is not a beginner-friendly task, to put it mildly.
To find the main function in a binary: first, note that tools such as IDA do this automatically. To do it manually, first familiarize yourself with the CRT functions that execute before main, find where it calls main. Then, for any given binary, you can locate the address of main by examining those CRT functions around the locations where they call main.
