Hi, not familiar with this particular CMS but I would start here:
Learn – Guides – Symphony.
I guess from what knowledge I have with other CMS's that it really depends on what you need to change.
Example, if a particular theme gives you the content you require but not in the style or format you require, the area of change will be primarily the CSS files associated to the pages you need to change.
If however you need to do more than change the colour\text\layout like adding more links or a new DIV etc you will need to start working with the HTML and in this CMS's case maybe the XSLT (but I know nothing of XSLT and its function with this CMS)
The first challenge I guess is working out what files (CSS\HTML\XSLT) connect to each other and what info they hold, make a change, view that change etc.
Install it on a test server and go through the tutorials, this is really the only place you can start IMO