How to compile .c .h .so file?

Another company gives me three files(demo.c,api.h,libapi.so). they told me that't enough for develop. I'm confuse how to build these thing together. I use gcc demo.c -o demo -l libapi.so . But's it's said "ld: library not found". I use Mac OS system. Some websites said i should use Linux to use .so file. what should I do?