I only know C and never used any game engine (i’m a noob). My friend said unity 5 only Works with c#, not c or c++, is it true?
Yes. A Unity version of Javascript is also available, but C# is the only built in C language.
There are two languages: C# and UnityScript (or as they call it: Javascript for unity). I don’t use C or C++ but I hear you can use a wrapper to use C++/C code in unity anyway. But you’ll have to create DLL files and other stuff. You can find out more in this section of the manual Unity - Manual: Plug-ins. Still, if you aren’t very experienced it may not be too easy to do (but I’ve never made one myself so I’m not sure).
Finally, I wouldn’t worry too much about it. C# is based on C and C++. C is considered harder, really. C# has a lot of semantics and structure from both those languages and it’s easier to grasp too. So if you know C you should just give C# a go (compared to the other two it does a lot of the work for you). The only thing you’ll have to get used to is working with Object Oriented Programming, which C is not.
As a last note, Unity is build on C++, not C#. Just in case you are worried about the engine’s speed.