Open up your theme's style.css
Find this.
body{margin: 0; padding: 0 0 100px 0; font: 12px Arial, Helvetica, sans-serif; background: #303237 url(images/bg-test.jpg) repeat-x; text-align: center}
Update to..
body{margin: 0; padding: 0 0 100px 0; font: 12px Arial, Helvetica, sans-serif; background: #303237 url(images/bg-test.jpg) repeat-x; text-align: center;line-height:1.2em}
Adjust 1.2em to your desired line-height, or if you're looking to set line height a bit less globally across the site and just want to target text in posts, then add this into the stylesheet.
.post p { line-height:1.2em }
or (if you prefer pixel heights, then..)
.post p { line-height:15px }
Again, adjust as required..