plugin creation

You debug a plug-in by setting Acrobat as the executable. But you are probably getting ahead of yourself. There is a lot of work to getting the first plug-in to build and install.

how to debug with VC++ IDE to generage .api file?

See item 4.  The specific API to issue a message, to add to the event processor (probably a menu item) is AVAlertNote.

Plug-in creation needs things to be done very precisely, because the plug-in becomes part of Acrobat and has to be built in the same way. Here are some thoughts.

1. There may seem to be alternative ways to do some things, and experts might succeed. But getting plug-ins to work at all is so specific and demanding that for a beginner to make any changes to how things are done invites even more frustration. As it is, it may take weeks before a starting plug-in developer finally persuades their plug-in to load.

2. Plugins MUST be created using Visual Studio (Windows) or xcode (Mac). More than that, you need to use the specific version recommended in the documentation for the SDK you use.  These are complex tools with more learning curve than I'd like, even from version to version.

3. Write plug-ins for Acrobat NOT FOR READER.

4. Always use a starter project or sample to start with. Get one of these working properly before adding a line. Be sure you know what you expect them to do: for example a plug-in has an entry in ABOUT only if it puts one there; no other magic does this.

5. Plug-ins can run JavaScript; this may be a good way to build on the investment of time you have made. But to move beyond a certain point you may need to rewrite all using the plug-in SDK.

6. New to C/C++? These are treacherous language, full of ways for the beginner (and experienced programmer) to shoot themselves in the foot. Writing a user interface beyond menu items, tool buttons, OK alerts and yes/no questions requires knowledge of the platform programming methods of showing windows, dialogs etc. - a very large subject in itself.

7. There are thousands of API entry points. Read all the documentation you can, including all the introductory material. Get to know the "layers" of the API so you can focus on what you need.

8. An SDK is often forwards compatible, but never backwards. For example the Acrobat XI SDK on WIndows can make plug-ins for Acrobat XI which also work in Acrobat DC, but not in Acrobat X. But on Mac, the Acrobat XI SDK cannot make Acrobat DC plug-ins; these must be new projects. Adobe do not keep old SDKs available for long; this will block you developing for older Acrobats because people are not allowed to share an old SDK either.