Also make sure you don’t infringe any third-party rights, like trademarks or trade names. You can check this by consulting the Benelux Trademarks Register and TM view (an international trademark database) for trademarks. For trade names there is no register, so you’ll have to check for example online or in folders.
Furthermore, choose a short and powerful domain name, try to make it easy to remember, write and pronounce.
As for the extension, the most common extension is .com. Extensions for countries, for example: .be, .nl, .fr, .eu,… are also popular. Using a country extension provides information about the location of the business, which can be important for certain entrepreneurs and businesses. You can also choose something more niche like .coffee, .pizza, .immo, … Keep in mind however that not all extensions cost the same.