Yes you can tweet your own post. If you are using wordpress there's even a plugin that automatically tweet every time you post.
I just want to know how to tweet a post. Like we can see in some blogs "XX retweets". i know what retweet is but how to tweet it. Is it just posting an update with link to it?
Yes, that's one solution. Easier solution for your site if your site have rss feed is, use twitterfeed.com. You can automatically tweet any updates from multiple sites. It's easy to use and you can login with OpenID..