If your site is new then Google won't have established a suitable crawl rate. You can submit new or updated pages in Google Webmaster Tools:
Goto Health > Fetch as Google
Specify a URL or leave blank for the home page and click Fetch
Once the page has been fetched there will be another button Submit to index
Select "URL and all linked pages" and click OK
You should also consider submitting an XML sitemap if you have not already. Again, in Google Webmaster Tools, under Optimization > Sitemaps. You can also specify the location of your sitemap in robots.txt:
When your sitemap changes, resubmit it to Google. You can do this programmatically using an HTTP request. See the Google Help Docs for more information.
And then wait; it takes time. As godka states, Google will generally find and index your content without further intervention. In the meantime, write great content and get people to link to it!