If you want to add scripting facilities to your program, make sure you use an existing language. Python, PERL and scheme are all easy to embed in your applications and are sure to be faster, more powerful and less buggy than any language you write. There's also a good chance that users will already know the language.
Look into this early. It starts with a #include in the app's config files and before you know it you've been suckered into adding support for lambda expressions, nested modules and garbage collection!
Which language you choose is up to you, and depends on the application. However, many ROX applications are written in python and this is surely the best language for beginners (and possibly experts too ;-).
In fact, it's often a good idea to write the whole application in a high level language as you avoid the whole problem of compiling (many users don't have a compiler, don't have the header files, run on multiple architectures, have wierd compilers, etc). Usually only very small parts of your program will benefit from being written in C; most languages allow you to embed small C functions easily.