Localize text

I want to insert a title in my theme and make that title localizable. I DO know how to add the string in the POT file.
What I do NOT now is how to declare the text in my .php file to indicate it should be localized. That means, how to write the code.

I’m trying to literally use <?php $translated_text = __( ‘text’, ‘domain’ ); ?>

but that didn’t work. Do I have to create a new function or something?

I appreciate a simple explanation since I’m totally new to php and localization.

Thanks