ive always had problems setting up a mail server so ill just comment on the web server part of your question.
acid_kewpie is absolutely right about the DNS settings. If the company your purchased your domain name through does not host your DNS then you'll have to find another provider. I use easydns - www.easydns.com.
they can host your DNS and e-mail MX settings for your domain name. Its very cheap, easy to use, and they update their servers every 15 minutes. So if your ip address ever changes youre only looking at 15 minutes of down time (give or take)
On redhat 9 i believe the default apache server directory is /var/www
Go into your redhat Service Configurator (i believe its K-Menu - System Tools - More Tools - Service Configurator) and make sure that apache starts when your system starts. you can also start apache from this menu to make sure thats its already turned on.
Open your web browser and type in the server internal ip (192.168.xxx.xxx) It should bring up the default apache web server page. Try placing an html file in the /var/www/html directory and try it out. Let's say you have a test.html file, then you would go to 192.168.xxx.xxx/test.html and that page should show up.
from there its just a matter of moving your website content in to /var/www/html and getting the dns setup for your domain name.