how do i use $my_var in a template

[ryuuguu]

I am trying to get a variable set in the postbit template from a plugin, but it does not show.
I have

simple pugin

PHP Code:

$my_variable = "My variable HERE";
echo($my_variable); 


this is set as an active showtread_postbit_create plugin. I know it is active since I can see "My variable HERE" at the top of the page .

in the postbit template I have

My variable below here
$my_variable

I know it is active because I can see "My variable below here" on the page.
but "My variable HERE" does not show up.

Is this the totally wrong approach? Is there manual page that explains what I should be doing?

Cheers,
Grant