host webpage from my mac

If I made a webpage and lets say I had a domain name, is there a app that I would use to host the web page from my computer?

I don't understand this, why do you have to pay the domain search compnay a fee per year. Is there a gov site that you can register them yourself?

First you need to see if your ISP blocks port 80 which is the port necessary for sharing the files. You would need a static IP address otherwise you will have to update the DNS records everytime you get a new IP address.

You have to pay the registration company to do corespondence with ICANN. ICANN doesn't allow for privates to register with them, and the fees are pretty unifrom all over the place (depends on extra services etc.).

Hosting your site yourself will cause traffic on your line and if you're just using a normal computer, rather than a proper server, your site will probably be unreachable most of the time due to bandwidth.

Still, if you know what you're doing, go with what trpnmonkey41 said.
Also, you will still have to go through the pains of registering your computer with the DNS or no-one will be able to access your computer.

lets just say, i have port 80, and I used my mac pro. what software would I use?

You don't need software. Just enable Web sharing in System Preferences -> Sharing and put your site into the Sites folder in your home directory

so the files from iweb, i can just put in the home directory? Do I need to name it something special?

Where would I set the domain name?

Yep just put the files in the directory. You don't set the domain name anywhere on the computer, you need to get DNS setup to point the domain name to your computer

thanks