Like the topic says, I want to learn a language, but which should I
learn? Which language will be mostly used in the future? C++ or C#?
Which to learn? The answer depends on your situation, and previous
experience, which unfortunately you don't tell us.
Which will be mostly used in the future? Meaningless question really
as it depends on what business area you're talking about - software
engineering is a very broad church.
So the best I can do is give my personal impressions ....
If you are a 38-year old estate agent who wants a career change but
is not particularly technical, Java or C# would be a good starting
point. They are relatively easy and straightforward for people with
a modicum of technical ability to grasp.
If you are a 17-year old whizzkid who already codes in x86 assembler
for fun, then learn C++. C++ is extremely powerful and rewarding,
but it is also complex, obtuse and requires more up-front effort
to make progress.
Most people fall somewhere between these two poles.
C# evolved from Java which in turn sort-of-evolved from C++; Java
was specifically engineered to be more idiot-proof than C++ and
pushed C++ out of a lot of its previous niches on that basis.
C++ is still strong in areas where portability and/or performance
and/or a high degree of low-level control are important - eg.
shrinkwrapped applications, games, operating systems. C# and
Java compete for largely the same niche, and predominate in
bespoke enterprise applications ("company X need a system to
automate their such-and-such operation" sort of thing).
This is necessarily a rather crude stereotype of a more complex
picture, but you get the idea ...
If you know C++, you can pretty much do Java / C# straight off
the bat (I speak from personal experience here). The converse
is not true; a C#er would be utterly perplexed by much of C++
unless he does a fair bit of training / reading first.
Use the internet to get a feeling for C++, C# and other
languages (though note there are a *lot* of really crap
tuturial sites out there); if you want a career in programming,
search jobsites, see what jobs interest you and see what
skills they require.
HTH