Can you give me an idea as to what I should do to make better use of WordPress? Should I buy one of the third-party books? I know html pretty well, but have no experience with css or php.
I have copies of both the currently available WP books, and did not find them particularly useful. They are ok for straight-out newbies that rarely do anything with a computer, but tend not to be so useful for people who like to get under the hood and are generally comfortable with using software, etc. My opinion -- others may differ. But if you are truly baffled by the basic WP admin, and you've already been through the introductory sections of the Codex, then pick up the "WP for Dummies" book -- I found it better than the other one ("Quickstart"?? Can't remember the other one...)
Okay, I've gone to Appearance > Theme > Editor. Actually, I've been there before, but it uses a language that I don't speak.
So, taking sidebar.php as an example -- are all the modifications I want to make going to look like this? If so, how do I learn to do them? Do I have to learn php? Does everyone who uses WordPress use php?
1) If you are not designing your own theme, you don't need to know PHP or CSS at all. You can choose from many themes (see EXTEND on this site) and use them "out of the box." But not all themes are well-designed or thought-out, so bear that in mind.
2) There are a lot of plugins that third-party users have developed for WP. They often provide some function that doesn't come with WP "out of the box." Some are so popular that WP eventually provides whatever function people are looking for in later versions.
3) Although wordpress allows you to edit your theme through the admin panel, most experienced users would advise you NOT to. The files you can usually edit that way are PHP and CSS files. You are better off editing those in a third-party editor and testing them first. There are a lot of free HTML / CSS / PHP editors out there, or use a commercial one. In other words, edit the files on your regular computer and upload them via FTP when you are satisfied they are working correctly. And always back up all your files first.
4) Not every WP users knows PHP or CSS, not by a long shot, and there are widely varying degrees in expertise among WP users. It is very helpful to know some HTML but not necessary. If you are designing your own theme, you will need to know something about CSS, but not necessarily PHP, although it will help. Also, you do not need to be a PHP guru to mess around with the templates etc. -- you can learn as you go, and the Codex often tells you exactly how to accomplish a specific objective. (And just as often -- it DOESN'T tell you, so you'll have to Google or ask here.)
5) Read through the Codex. When I first started putting up WP sites, and learning how to customize them, I personally found the Codex fairly confusing and overwhelming, so you are not alone if you feel that way. But it is still the most comprehensive and definitive source of learning about how WP works.
But there are others, as well -- if you Google for WordPress tutorials, you will find them, and many of them have simple, straightforward instructions or tips.
I suggest setting up a test site, and messing around with it, the templates, the CSS, etc. to get a handle on things like template tags.
6) Be aware that this forum consists entirely of WP users. Very few developers post here, although there are some. So mostly it is just users helping other users -- no one is paid. Even the moderators are volunteers. So this is not the place for angry rants about how you don't like WordPress. And be patient when you post questions -- again, questions are answered by people just like you -- just people who have learned something about WordPress and want to share what they've learned.
Good luck. Personally, I really love WP, and as my knowledge has grown, I've been able to customize it more and more. But I have often been very frustrated, p****ed off, etc, etc. But it's free, it's being developed all the time, and it has a huge user base, so I'm committed.
PS IF you don't want to install your own WordPress site, you can also try out wordpress.com, which is basically just sign-up-and-go, although you don't have much ability to customize.