read - read - read 🙂
Read the forum / the boards here. That should get your started. If you know C already, you also know that starting with the easy stuff is hardest, once you master that, you can start building. sid is absolutely right about the datamodel. However.. I also know that if you have no experience in database design, you will most likely not get that right. So do not worry tooo much about it (But do worry ;-))
Start with:
- Getting a form to post to another page, and displaying the info on the next page. Look at $POST, $GET and print_r()
- Sanitizing user input, look at [man]mysql_escape_real_string[/man]
- Inserting, and retrieving data from databases. Just look at mysql.org and more specifically, this website, database forum.
From there you should start looking at sessions, create a basic login script, the use of CAPTCHA to reduce the number of spamm messages on your forum.
Have fun, and welcome to phpbuilder!
J.