The only part I modified was this
Well, there **should** be a setting somewhere so you can use the system's variable for that, something like
<title>{$mybb->settings['sitename']}</title>
but I've no idea if it's actually called "sitename." But if you did this, with the brackets,
<title>{Here is the name of my site}</title>
It's likely the code is "doing something" with everything between { and }, and if you just typed something in, it will cause an error because there's no variable "Here is the name of my site". You could give this a try.
<title>Here is the name of my site</title>
If that doesn't work, start over, re-install it fresh, make sure it's working, then modify but back up your files. :-) A good way of working, if you edit file.php, first save it as file-bu06-28-10.php and save it to the same directory, that way you'll always be able to revert.
An aside, modifying existing code is a really good way to get your feet wet in any language - so long as you make backups. :-)