I know there are many search engines like Google, Yahoo, Bing etc.

I want to optimize my site mainly for Google, then what should I do so that my sites would have good ranking in Google?

>Forum posting
>Blog commenting
>Social bookmarking etc.

hi.
Google likes manual work and unique work, no spam. Secondly you can do forum posting and social bookmarking, that will surely help you.

For a high SERP position in Google, you will have to employ rigorous and consistent on-page and off-page optimization techniques. Here's Google's very own beginner's guide to help you begin with:

http://static.googleusercontent.com/external_content/untrusted_dlcp/www.google.com/en//webmasters/docs/search-engine-optimization-starter-guide.pdf

make your page content unique and quality
and then to build backlinks for your site

Use different on page and off page optimization techniques, use unique content and update it frequently and do quality link building for different pages of your website. To get better result in Google always keep in mind one thing that never use any black het techniques always do white hat SEO.

hi.
I mostly do Forum Posting and social bookmarking. Secondly i prefer only unique content and good quality backlinks.

The first step is to do a thorough keyword research, then build backlinks from forum posting, articles, etc targeting those keywords.

Google likes manual work and unique work, no spam. Secondly you can do forum posting and social bookmarking, that will surely help you.