There are a few ways to decide on a domain name for your website. Most often, when possible, using the name of your company or organization is the best choice. I recommend getting a .com for a business or a .org for an organization. These are easy to remember and until your get a high SERP (search engine results page) a business card may be your most effective way of promoting your site. Since domains a inexpensive I will often register a few to get going, especially if the one you want is already taken.
Sometimes registering a descriptive domain name is a good option. I registered rentalannarbor.com for my rental, it is not the name of my business but it is descriptive and easy to remember. Having multiple domain names lets you direct traffic to specific parts of your website, an inexpensive way to build links.
When I stat a new blog I tend to let the content drive the domain name. I have dozens of domains registered for specific projects. Occasionally, I will sell or rent one to a client to get them started, and once they are comfortable creating content and managing a website they have a better idea of the permanent URL the want. Once you have a domain you can take the next step which is selecting a host for your website.