Already Own Your Domain?
If you already own your domain, it is important to remember that you must go to the website that you bought the domain from to make any changes to that domain, including buying an ssl certificate. For instance, if you registered your domain at www.godaddy.com, then you can't buy the ssl certificate from directnic. You will need to buy the ssl certificate from the same website that your domain is registered with.
Need To Buy Your Domain?
First, you need to decide where you would like to buy your domain. Whichever Domain Registrar (DirectNic, GoDaddy, Network Solutions) you buy your domain from, this is also where you will go to make changes to that domain. So, it is very important to remember where you bought it and what your account information is. In this example, we will be purchasing a domain at DirectNic which is located at http://www.directnic.com/.
Create an account at DirectNic if you do not already have one.
After you have created an account, you will need to login with that account and then follow the instructions on their website for domain registration which is located at https://secure.directnic.com/help/guides/index.php?guide_id=1.
You can ignore all of their hosting services documentation. All that you are interested in is registering and paying for your domain. When purchasing, leave everything set to the defaults for DNS settings. You may also want to consider buying the domain for more than just a couple of years. If you only purchase the domain for 1 year, you run the risk of forgetting to renew it and having your website go down 12 months after it is up. The fees for domain registration are nominal; but the hassle of trying to get an expired domain registered again is not worth it if your regular customers get the impression that you have gone out of business.
See the links below for specific instructions for DirectNic and GoDaddy, two of the most popular Registrars.