If you haven't written any php code before - try this on your development server or a test site first.
The process is fairly simple.
1 - Click on the Create Plugin icon at the top of the left menu
2 - Give the Plugin a name - something that you will remember
3 - Paste $modx->loadClass('sources.modMediaSource'); on the line below the <?php in the text box
4 - In the 'System Events' tab scroll down to 'OnHandleRequest' and click the check box
5 - Save
Step 4 is the critical one. It binds the plugin to a system event. Without it the plugin doesn't do anything.
There is more info on plugins at
http://rtfm.modx.com/display/revolution20/Plugins