-Where do I actually type my php code? In notepad and upload it,
or somewhere else?
PHP code is inserted into HTML documents, either in the head or in the body section, wherever it is needed (similar to javascript). The code is marked by special PHP-tags.
-What files should I upload to Tripod?
The pages of your site, like before, only they should have the extension .php instead of .htm or .html, to signal the PHP preprocessor that there's PHP code in there. Plus maybe included files. And, if you use a database, tables have to be made and populated.
-What is my "localhost"?
A name tripod uses for its MySQL server. Needed to connect to that server in your scripts.
-What will need to be edited for php/MySQL to work for me? Not depending on my project, just what has to be edited for the bare minimum functioning?
Sheez! That's an unanswerable question, since it ALL depends on what you want to accomplish!
-Since I'm very clueless with this type of language should I learn another before I learn php? I know so far, JavaScript, DHTML, HTML, and CSS, the basics.
What you know sofar will let you make pages, dynamic pages and client-side scripts. PHP will add the power of server-side scripting. Besides PHP there's also CGI, ASP, JSP, etc. but PHP is up-and-coming and is said to be easily learned, one of the reasons i decided to delve into it. It's also open-source software, and therefore free, which makes it very interesting for hosting providers to provide it to their customers, whereas most other server-side solutions cost them (and you!) a lot of cash.