How to edit the template file?

In WordPress you can upload and activate plug-ins easily via the admin log-in screen.
But where and how do I edit my template to make it really works?

For examle the weather-plug-in (see below):
5. Edit your template file and put the WeatherIcon function where you want your weather data to show up:

<?php WeatherIcon(‘station=KLFK&before=
• &after=

‘); ?>

Where do I put this???

Thanx!