how do i compile c programs in xcode 7?

It depends what you mean. To compile a C source file, you give it a ".c" extension. You can then add it to an Xcode project, and it will be compiled as C when you build. Or, you can manually run the 'clang' compiler from the command line (e.g. Terminal).

If you're asking how to create a C program, then again it depends on what you mean. If you want a macOS (or iOS, etc) app, then you start with an app template in a new Xcode project**. If you want just a unix-style command-line app, then you can start with the corresponding template in Xcode, or you can do it with the various standard unix tools.