Topic on Talk:Wikibase

• Construct an ApiMain object with the arguments passed via the URL and call its execute() method to do the actual API stuff.

I'm afraid this question is way to generic. I can see it's a quote from Manual:Api.php. But the relevant question is, what do you want to do? What do you have and what do you want to achieve? Maybe it helps if you start playing around with the d:Special:ApiSandbox?