There are differnt ways how Your site can be indexed in the SE's like google,
They just need to find Your site. This can happen through incoming links from other websites, through submitting a sitemap to google or others or simply by adding Your site to the google webmaster tools which are very helpful.
To make Your site be found fast, the best way is to submit Your sitemap to google, q2a features a sitemap plugin in the plugin section in admin menue.
The prozess of spidering Your site is either by reading this sitemap or by finding Your sites pages through incoming links and following all internal links on Your website.
If You have own, not copied questions and answers on Your site, its very probable that google indexes the largest part of the pages You have. If You have many errors or duplicate content, google will add less pages and rank them lower...
The big secret of search engine optimization (SEO) is incoming links.. Search for SEO in the web and read a bit about. If this is Your first website, I am sure someone here will send You a (starter) link ; )
For further questions just ask..