By community website, do you mean a website like Myspace, Youtube, or like this forum? It could be done using various different ways which depends on the choice you choose which all of them, require a MySQL Database for user data storage and other things that would make such things possible. Find a host that supports PHP, ASP, and has a MySQL Database, maybe more than one available for your use.
There are many different types and kinds of forums, free for use, that you could use for your community forum. It is advisable that you use the skin for the forum that you choose to have on your website. Preferably one that you have made yourself rather than those pre-made themes that you can install on the web. I can help you with that since I just two days ago figured out how to change the skin on a phpBB forum. To be more specific, I learned how to change the skin on phpBB3 RC5.
To make a community site such as MySpace or YouTube, you would need extensive knowledge on PHP. Generally, you need good knowledge in programming field or have a freelancer do it for you. That type of thing would require PHP and maybe ASP, depends on some things. I don't remember which is more secure, PHP or ASP. Somebody here might be able to tell you that.
The way I learned it is from making amateurish sites and than continued my way up. Not that I still do professional job on web design but it was better than it was years ago. I started off with a plain color background and information, than I learned organization on the web using tables (bad idea, start off with CSS, the higher beginning grounds), and than I discovered how to make web sites with image backgrounds, and than I figured out how to make CSS web sites, and now I'm discovering the secrets behind PHP programming

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Generally, I would say start from the ground and go your way up. When you first get a job, you don't begin as a manager, supervisor or a president. You start off as an employee receiving your share of the income, and over time your income increases and you might get promoted on the job, which increases your income as well. What I'm saying is that when you first start of on web designing, than you start by making amateurish looking sites, than you learn how to make good sites and because of that you are rewarded with better looking sites, than you promote yourself to CSS or PHP, whatever it is, than you are able to make better sites which yield better rewards for you. See the resemblance in this?
But I'm not saying to start of by making bad sites and than making good sites, I'm saying that it may be likely that you, on your first attempt at making your professional site, that the first result may be not what you wanted to be when you started off and that you may need to restart on the project. Right now, I'm discovering a way to make a very easy editable way at making a site that when you edit one page (like the menu for instance), than all of the pages that you have are edited, so you won't have to edit 500+ pages to fit your preferable new layout if you would want to change it in the future.
I'll advise you to start learning how to create CSS websites since they are most flexible technology of the present that I can think of at the moment. Technically, IE does not agree with Firefox and other more "better: browsers there are on the web so you may have difficulty making them IE
and Firefox compatible, but over time you may learn the secrets of IE
and Forefox compatibility. Also, don't forget Opera
IFind a tutorial on CSS. Below I listed a website that lists all of the CSS codes that you can use.
CSS1 Reference
CSS2 Reference
I sometimes use them to aid me on my CSS creation.
Hope that this somewhat helped you.