How to "Get The image" plugin works?

Start with reading the “How do I add it to my theme?” section in the FAQ: https://www.ads-software.com/plugins/get-the-image/faq/

That’s the basic usage. However, there’s more in-depth usage here: https://github.com/justintadlock/get-the-image/blob/master/readme.md#usage

If this is too much code for you, feel free to drop by my support forums. We’ll see if we can get you straightened out. However, the best course of action is to ask your theme author to add in support for the plugin.

Where exactly do i need to add this code in theme ? single php or functions.php ?

It really depends on the theme itself. Because all themes or different, you might have to add code to one file or many files.

Drop by our forums and link up your theme. I’ll be happy to help: https://themehybrid.com/board/topics