Editing PHP

Hi There,

I want to augment the tagline and I want to put line feeds in it. Problems:
a) HTML tags inserted into the tagline don’t work
b) Changing the tagline makes it long – I only want to change what appears under the blog title.

Reading old help articles suggests that I need to edit header.php. However I’m guessing the interface and tool set has changed since the articles I found were written, so how can I edit header.php to make the changes I want?

Step by step and from the very top would be appreciated.

TIA

Bob