See this section in the MediaWiki API documentation, specifically involving getting the contents of the page.

use the sandbox to test the API call.

These are the key parameters.

rvsection = 0 specifies to only return the lead section.

See this example.

http://en.wikipedia.org/w/api.php?action=query&prop=revisions&rvprop=content&rvsection=0&titles=pizza

To get the HTML, you can use similarly use action=parse

• http://en.wikipedia.org/w/api.php?action=parse&section=0&prop=text&page=pizza