That all depends on where you are and what countries you are focussing on. In Europe and the US over 80% of all searches are done on Google so you tend to focus on Google.
If you are in China then Baidu is by far the most popular search engine, so you would focus of Baidu if you have a Chinese website.
If you want to know more about SEO then you should look at website like
www.seomoz.org which is packed full of useful info. SEO is continually changing as search engines change their algorithms to make their searches more relevant so you need to keep up to date with the latest trends.