Scenario:
"I am a new Nucleus user and have just set up my blog. I would like to edit how my main page looks, but I can't seem to find the file!"
Solution:
Nucleus has 8 different types of pages, known as "skin parts". The main page of your site is controlled by a skin part known as the "Main Index" skin part. To change things on your main page, you will need to change the code that makes up the Main Index skin part.
Original forum thread (thanks, admun!):
http://forum.nucleuscms.org/viewtopic.php?t=4100
Method:
You can edit your Main Index skin part by going to your Nucleus administration area and clicking on "Skins" in your administration area menu:

Next, click on the "Main Index" link of the skin you are using. Nucleus uses the default skin when you first install it:

Finally, you will see a screen that has a large text area with a whole lot of code. You can click on any of the little
icons to get an idea of what the special Nucleus <variables%> do. For all the other code, you will need to refer to a HTML reference or guide. Make any changes you want here:

When you are done, remember to click "Update Skin" to save your changes! If you have made a mistake, you can click "Reset Data" to restore the code to the last save. Remember this will wipe all unsaved changes, so make sure you really do want to reset!
Further reading: