John, your question suggests you are right at the beginning of this process. Unfortunately the necessary answer is incredibly broad, but I'm going to do my best. I'll bullet point considerations you should make, roughly in order.
Identifying Useful (and prudent) shortcuts
- Can you get around needing to build a chatroom by using an existing chatroom service? I'm sure there are many services out there where you can take existing chatroom code and reuse it, or host your chatroom with someone else.
- If you do need to build it yourself, how much of your idea/requirements are truly unique? If there isn't too much of a difference, it's often best to customize someone else's code rather than building from scratch.
General Considerations in Designing your project
- How much time do you have? How much money? What quality level do you need?
Technologies to consider
LAMP is a very common web-development framework, which basically means Linux, Apache, MySQL (or equivalent), and PHP/Python/Perl/Ruby on Rails/Java. .NET is an alternative to this list. If the project is significant, I'd suggest taking a good look at the tradeoffs between these options and making a decision based on what feels right to you. PHP is the most common language, but Python and Rails (my favorite) are arguably more elegant. Any of these three options are completely solid and will put you in good shape, with strong online communities and plenty of support.
Getting Started
I was amazed by how rich the resources are online, and many free and high-quality. I got started in Rails by getting an intro book from Borders for a nice cohesive crash course, then I used SO, Railscasts, and other online resources to answer questions as I got them. Just remember to keep your questions specific and limited so future answers can be more concise and sharp than this one!
Best of luck with your project.