Anyone recommend softwares which use c/c++ for their extension development?
There is a whole world of software that uses c/c++ for their extensions, it all depends on what field you are interested in. For example if you are interested in music and DSP, you can create VST instruments, which extend music applications with VST instruments. VST instruments and effects compile to dll files. If you are interested in graphics, you can write plugins for Photoshop, or Adobe Premiere.
I also found several similar question about how to earn programming skill, and most are answered to participate in opensource project. However I found these projects too huge, as a beginner I don't have any clue where to start with.
If it helps, there are plenty of open source projects that are small, in fact one might say that there are far too many. Games are often a good starting point, in fact just working on a tetris clone will teach you a great deal. When I used to teach people programming, I would always suggest they begin by creating a tetris clone. Board games are great too, and you can do them all in the console
so you won't have to worry about graphics libraries and all that jazz. Excellent starting points are Monopoly and Clue. Card games are good as well.
I know your question was about plugins, but I given you mentioned that aspect it seemed necessary to point out.