This widget is one of the popular ones related to Facebook on blogs..To have one, you must first have a facebook id.
After logging in to FB, there are two ways to creating a page..
First method --
Login to facebook
Click on "create a page" button..
After this the steps are common for both the methods ..So lets look into the second method also and then the common steps
Second method --
In case you are not able to locate "Ads and Pages" on the sidebar, go to http://www.facebook.com/pages/. Login with your facebook id and password.
From now onwards the steps are common for both the methods
The page that opens up is
Select the appropriate option under official page and provide name for your page. click on the option if you are the official representative of the page..
Your page is created..You can now fill in more details like providing details of your blog, adding picture, inviting friends etc..
To add a widget on your blog suggesting readers to "Like" your page..scroll down the page under the "getting started" option..
Click on "Like button" option
Enter the url of your facebook page under "facebook page url"
Change other columns as required and you can see the preview after every change in the side box.
Finally click on "Get code" and a window opens up with the code.
Copy the code and open blogger..Go to design tab and click on "add a gadget". Select "HTML" option and paste the code there. Save. If you wish to you can change the location of the widget on the blog.