How to use Web API ? (About using last.fm API)

I'm new developer i want to ask about using last.fm API generally and using it with C specially. I never used API's befor

1-Steps: I can't do the first step in code , where do i begin to make authentication. ( i just made an API account).

2- Advices: Please give me your advices, i feel lost in the project.

3- Should I use any other implemented lib ?

Thank you for your help.