Getting your website online is one thing, making it appear in search engines such as google is another. A good start is having a look at the Google Webmaster tools.
Although most search engines automatically can index your sites, they all appreciate a sitemap that reveals all relevant pages to be indexed. This is particularly interesting if your site is new and does not have a lot of external links referencing to your web pages. More information can be found here.
The robots.txt file has 2 primary functions