Go to Settings > Search Settings and Enable Meta Tags.
In the Text area give a 150 character description which will describe
your Blog. This text might be used by Search engines when your Blog’s
home page is displayed on Search Engines. Here is the snippet that I
have given “Blogger Widgets provides you the best quality blogger
tutorials.It also provides you with free blogger widgets and gadgets
to build a better blog.” 
Now when you make a Post, you can set the Meta Description from the Post Editor’s Post Settings Sidebar.
You can always edit the Search Description for existing posts as well
If you are having a custom template, make sure that the meta tags are rendered by your Blogger Template. To do that go to Template and Proceed to Edit HTML .
Click Ctrl+F and find <b:include data='blog' name='all-head-content'/>
If it's not present, add it just before </head> and click on the Save Template button