Have you signed up for Google's webmaster tools (google.com/webmasters)? It would be hard to tell you exactly why Google isn't indexing your site, but here are a few things to consider.
If your website is relatively new or small, it is possible Google doesn't know about your website yet as it relies on backlinks to discover new sites. If this is the case, you need to submit a sitemap to Google.
If Google is aware of your website, you can check individual links to see their status and get a report for why they are choosing not to index it. In some cases, you just need to manually add it to their queue, but in other cases, you will have to address what is their report.