PC development (general)? C++
PC development (microsoft software eg databases/spreadsheet macros etc)? Visual Basic/C#
Mac development? Objective C
Oh, and learn basic assembly language for a simple target (68HC11 comes to mind). It will give you an insight to how a computer really works more than any other language, or study, can.
As for certification - if you're serious, consider a computer science degree.
Well, most of your COTS (Commercial Off The Shelf) Software is coded in C++, so if you mean writing PC applications, as in heavy duty, solid Windows applications that could be sold professionally OR working for a software house like Intuit, Symantec, Microsoft, etc then C++ should be your choice. If you mean working for any company in their IT department doing desktop development then .Net, ASP, Java and probably before any of those, learn PowerShell and some VBScript (Visual Basic). There is no right answer here because it's like asking what language you should learn to speak, that all depends on what countries you plan on visiting. As a Senior Systems Engineer that's been in the IT field for the past 20+ years, I can say solid scripting is where you should start as it teaches logic flow, subroutines and error handling, something common to any/all programming languages. If you have an aptitude for that, continuing on will feel natural...