Schema.org markup can be added to most sites without a plug-in. Assuming you mean a WordPress site you should be able to add the code in using the html version of the text editor.
Adding sitewide code is fairly simple. You just need to edit header.php in your theme.
If you want each page to have different schema data you will need to create a function in functions.php and a hook to add the resulting code to the document head.
It’s not possible to give a meaningful answer without knowing exactly what schema data you want to add.