For applying a patch you need to be able to compile Dosbox. That's the first thing you need to solve. Instructions are in the wiki.
Because a patch is normally for the Dosbox sources, not the binary. A patch should not be a *cpp file. Cpp files are either replacements for source files or additional source files.
A patch normally is a file with diff or patch in its file ending.
To apply it you need the "patch" program and use it on the commandline, mostly like this:
patch -i patchfile.diff -p0 sourcefolder
(The -p switch is most of the times necessary but don't worry right now)
For old patches you will need to manually adapt source files since most of the times dosbox changed since the patch was made..

So, first get to compiling dosbox (and first all the libs needed for dosbox) and when you succeeded with this, ask for further help