This post is going to cover the most basic aspect of your company's online identity: your domain name. Most people know most of this already, but I think it makes sense to start from the beginning and work our way to more advanced topics later.
So first, what is a domain name? It's basically just the address that is used to access a website. For example, Yahoo's domain name is "yahoo.com" or "www.yahoo.com". If you want a website, or an email address that ends with @Your_Company.com, you have to buy a domain name. They only cost about $10/year, so there's no reason not to have one.
It's important to know that a domain name has nothing to do with website hosting. You can own the domain name example.com, but that doesn't mean you have a website. Many companies sell hosting plans and domain names together, but you can choose to buy a domain name on it's own. This means that even if you don't think you'll have time to get a website set up in the near future, you should buy the domain now so that you have it. You can worry about what to do with it later.
Now let's get into the different parts of a domain name. A domain name is generally split into three parts: (1)www . (2)yourcompany . (3)com. I'll explain them in reverse order.
(3) Top-level domains
Every domain name has to end with a top-level domain. There are different ones to choose from, but the most common ones (in America at least) are .com, .org, and .net. There are other options to choose from, but when you buy your domain name, the top-level domain has to be selected from one of the options that already exist. You can't make up your own top-level domain.
(2) Second-level domains
The second-level domain comes between "www." and ".com" so this is where you get your real customization. Many of these names are already taken, particularly with the ".com" top-level domain, so you may need to get creative to find a good domain name that is available.
The sub-domain comes to the left of the second-level domain. For example, most websites use "www" as the sub-domain. You don't actually need to pick a sub-domain when you buy a website, because if you buy "example.com", you automatically own all sub-domains of that (www.example.com, blog.example.com, etc.).
Finally, there are some odds and ends you should know before making the purchase. Domain names aren't case-sensitive which means that it doesn't matter how you capitalize it (example.com is the same as ExaMPle.com). Also, they're so cheap that you shouldn't wait to buy one just because you can't decide on a name. Buy one now to get started, and then if you think up something better, buy that too.
Now you know way more than you ever wanted to know about domains. If you don't already have a domain name, it's time to buy one. There are all kinds of domain registrars out there and they all do pretty much the same thing. Godaddy.com is the most popular one, but I personally have been using Dreamhost lately. As I mentioned above, you can buy a domain name from anyone and host the site elsewhere so it really doesn't matter where you buy it.