iWeb website using already purchased domain

I want to try to create a website for the small business I work for using iWeb. We currently have a domain name and website, but the website is not really up and running like we would like it to be. I recently purchased a macbook and this is my first mac so I am not completely familiar with everything...also this will be the first time I've ever attempted to create a website. The iWeb program seems fairly easy to use but I am having trouble getting iWeb to connect to the domain name that we already have. I put in the username and password that I have for the domain but it is not working. What specifically do I need to do (in like, computer for dummies-type terms please!) I have a pretty good knowledge of computers but when it comes to all of this website stuff it's like I'm reading a foreign language! I've looked into the domain account and there is something about locking and unlocking the domain. I've also read a little about free web hosting as well. I just feel a little out fo my league here but Im deternmined to figure this thing out!