First, you need to buy a domain name.
Then you go and get hosting from somewhere. Here, you'll want to know a few things. First, what the storage and bandwidth quotas are (the two numbers you quote). You'll also want to know what databases are supported, what server-side languages, etc.
Then you'll get a list of domain name servers (ns1.host.com or similar), and you go to your domain name's control panel from wherever you bought the name, and change those values. That points that domain to your host's computers.
The quota requirements depend on what you're doing. Are you going to be selling things? Are you going to be using lots of large images? Videos?
100MB is more than enough for any basic website with simple pages -- thousands of them. However, if you're delivering lots of high-res images, then this may not be enough to store them.
The above paragraph also applies to the bandwidth too. If you're just delivering simple pages, 1500MB is fine. If there's lots of images and larger file type like that, it may not be enough.
Finally, are you going to be selling anything though the site? Getting a system to sell things can be quite complicated or involve more money. The SSL certificate alone is going to cost you a yearly fee. You may be better here to use paypal, which will do the hard work for you for a small cut.