Unix APIs are all C. Unix and C are best friends. Other languages essentially all end up using the C routines for getting at the system. For example, perl is in C, the JVM is in C, etc. Any call to a UNIX API will be the description of the C call. Obviously things that can call directly into C can use the APIs as well.
Essentially you will find out what methods you need, and look at a description in a book, the man page, or something online. You'll need to find out what header file (.h) you need to include to use the function, and what types you have to pass in. Many system functions use pointers to special structs as arguments. Normally these will be defined in a .h file as well.
If it's not your forte it might take some time to adjust to some of the UNIXisms, like "everything is a file". When you open a socket, it's a file descriptor. You just poke at it in slightly different ways.
good luck!
-Lee