As a beginner you're a long way off from creating cross-platform apps. Perhaps even mobile apps in general. I suggest targeting desktops initially, pick a platform you're comfortable with and focus on that.

I wouldn't recommend C/C++ to a beginner. They are well worth learning but not good beginner languages.

C# is a good language in general (perhaps my personal favorite) and cross-platform thanks to Mono, at the cost of a license fee. Java is similar in style to C# and happens to be the language of choice for Android apps.

Python is often touted as a good beginner language and I agree, it's a pleasant language. Somewhat tricky to make mobile apps with though. But definitely a good choice for learning the skills of programming (hint: programming is a transferable skill between languages - the one you learn initially isn't the be and end all. Any developer worth his salt knows dozens of languages, frameworks, etc to varying degrees of proficiency).

Then there's HTML+Javascript which is arguably the most portable solution; lots of apps are really just HTML5 and Javascript; you don't even have to make an app as such, just stick it on a webserver and then any device with internet access can get to it.