The first step is to settle for the domain name.
Key pointers that will help you find an accurate name include the name of the brand, location registered in, extensions available to you etc.
When you are armed with adequate information that will help you choose the domain name, find a domain name registrar and try out some variations. Most of them will give you dozens of variations that could be considered.
When you have found the right name to use, you can proceed to registering the domain.
When you have registered the right domain, you can finally proceed with your branding and advertising campaigns in confidence. It is important to note that even though most people are used to .com and .net domains, you should avoid limiting your options to these extensions. Extensions that allow you to basically have a two word domain name can quickly stick with users in the same manner as the .com extensions.