This plugin is really a collection of a lot of useful components for your blog. This includes these basic functions and many more:
accelerator tools to speed up your blog
analytics to see how many people visit your blog
Google Adsense, Paypal and many other e-commerce options
related posts beneath your posts, very useful as your blog grows
With the Basic blogs Jetpack will be in your list of plugins. You'll want to click to activate it.
For Advanced blogs you'll need to add the plugin. Search for it, add it and activate it like this.
Click to Set up Jetpack
You'll be taken to Wordpress. If you already have an account login. If you don't click to create an account at the bottom.
You'll need to click to Approve the set up. Make sure the blog is linked correctly.
Once set up, it will give you various options for Jetpack. For the Basic bog you must choose the free option at the bottom. For Advanced you decide which option best suits your needs.