How to call a plugin function from index.php

The same way you would any other:

foo();

Active plugins are loaded before the theme files

You may want to check that your plugin is activated and the function is available so things dont go pear-shaped if you forget to activate it, like:

if(function_exists('foo')){
    foo();
} else {
    echo "oh dear you haven't activated/installed 'myplugin', go do that before the 'foo' feature is available";
}

Also keep in mind foo is a very generic function name, perhaps the “omgfoo” plugin also has a foo function. So prefix/namespace your function to something unique

You will eventually want to use actions and filters, as they’re safer and better practice, you can continue to read up on that here