goodness me you are adventurous.
I'd start off by saying that designing something like a forum is not a simple task - i'm not saying that you shouldnt do it at all, I'm all for plunging in and learning by example. However, it sounds like you might have dived in a bit too far - php and asp's create dynamic web pages: they get away from the static mark up of HTML and move towards code driven automated sites. That's why everyone loves them, because they give so much more flexibility and are just good fun to tinker with. if you want to start doing your own dynamic sites, then i would strongly strongly strongly suggest that you get some sort of web server on your computer: this is the kit that "runs" your web page - you cant just design a page and then double click on it with php and asp, you need something to "run" it since it contains lots of code and stuff. If you have win XP pro then you're off to a flying start since this comes with IIS, the microsoft web server that will run your ASP's. If you want to do PHP's, then you best head on to PHP.net. They have all things php there, including how to install PHP into IIS, or if you dont have IIS it will give you links to Apache, the free web server that you can use as an alternative. While you're there, get MySQL too cos you'll probably (actually, definitely) need a database if you want to do a forum.
Then your best bet is to get a book that tells you all about it - I personally like SAMS, and they do a fantastic book on programming with PHP and MySQL - i think they do ASP ones too, but it depends what you like. I think Wrox are supposed to be good too. Yes you could do online tutorials, and sometimes these are jolly good, but personally I like to be able to flick around a really thick book, see what's coming up, go back to pieces before etc - it's just not the same when you're flicking between browser windows on a screen.
The reason i suggest getting one yourself because you#'re going to be much quicker at coding if you can do that, rather than having to upload it every time to your server to test it. as i said before, you cant use PHP or ASP without some form of web server. And on that note, you need to check whether or not your internet server allows you to do either PHP or ASP - not all do, and many charge extra for the proveledge. You may find in that case that you're better off not bothering, just playing around with it yourself at home.
On the question of why they all look the same, its true that there are generic scripts out there that you can download and use (some free, others not), but i think the main reason is that they all do much the same thing, and there aren't too many ways to skin a cat!
best of luck with all that - dynamic web sites is a really fun thing to get into, and if you've done a bit of HTML then it really will get exciting from here on in!