Once you register a domain name, you will be able to modify its name servers. Those are addresses that associate this domain name with your web hosting account. Once you open a hosting account, you will need to modify those name servers to match the names that your new host will provide. Information about the servers will be included in the Welcome email you get when opening an account.
Once your hosting account is activated and you modify the name servers, it will take around 12-24 hours for the changes to propagate across the Internet. Once this happens, your new web site will be accessible by using your domain name. You will usually be able to upload and access your web site even before using the control panel or just the IP address that your host will provide propagates the name server changes.
