Read More… option on posts

I would like to set up my weblog posts so that, especially for longer ones, only half or less is visible, and you click “Read More” to view the full post.

This doesn’t seem to be an option in my Sirius theme… is this a standard option for WordPress… or does it depend on the way the theme is written? Any suggestions on how i could get started setting my blog up this way?

Thank you.