In order to have a counter on your journal sidebar, you will first need to find a site that offers them. I personally use
easycounter.com. They give you an html code to copy/paste and the whole process is very simple.
There is more than one place on the journal you can add a counter. You can put it in a
custom module box or you can replace an exsisting module box (ie, the syndicate module).
Custom Module
If you are placing the counter in a custom module, follow the instructions in the
adding custom modules tutorial and place the html code where you would place any other custom text.
Replacing "Syndicate" module
Use the following code in a theme layer to replace your "Syndicate" module with the web counter:
function print_module_syndicate(string title) {
var Page p = get_page();
$title = "Web Counter title";
open_module("syndicate", $title, "");
"""
Your html counter code here
""";
close_module();
}