1. What’s the purpose of the domain name?How are you planning to use this name? If the domain name will be your primary company website, try finding the closest version to your company name that you can. If you're just starting out, choose your business name and domain name with care. Make sure your visitors know what your domain name means or, if it’s an acronym, what it stands for.
2. Brainstorm a list of ideas of the problem you're trying to solve or the solution that you have.
A domain name that clearly indicates what you do, or a problem that you solve, or a solution that you have to a problem will give a visitor a fairly clear picture of what he'll/she’ll find on your website.
3. For SEO (Search Engine Optimization) purposes, it helps to have your keywords in your domain name.
Before buying your domain, make a list of keywords that someone might use to find you online. This list could include your industry, your target market or niche, a problem your target market has, or a solution that you can offer.
4. Shorter is better, if it's to be your primary domain.
If the domain name is going to be your primary domain where your primary email address will be housed, you want your domain name to be as short, catchy, and memorable as possible. After a few times of spelling out your lengthy email address, you'll come to appreciate the beauty of a short domain name. Your domain name can contain up to 67 letters and numbers, although I would encourage you not to have one of this length, and can contain no special characters other than hyphens.
5. COM, ORG, NET, etc?
One common question I encounter is from people who can't get the ".com" domain of their choice, but find the ".net", ".org" or other country-specific top level domains (TLDs) available (like .de, .nu, .sg, etc). Should they try for these?
The answer is not as straightforward as you might think. If your website or business caters to the local community, such as a pizza delivery business or recruitment agency or the like, then it makes sense to get a country-specific domain. You actually benefit from having such a local domain because the people in your country know that they're dealing with a local entity, which is what they want. After all, if they stay in (say) the United Kingdom, they're not likely to want to try to order pizza from pizzaparlour.com, which suggests an international site. You'll have better luck calling it pizzaparlour.co.uk, ie, with a UK domain.