By Giselle Ayala Mateus
First, if you are a sole-proprietor you have the option of doing business using your legal name or using some other name that is more "fun" or that is commercially strategic. If you will not be you using your legal name you need to register your trade name or d/b/a. Keep in mind that the name you use to identify yourself when entering into contract is just a name for conducting business not a trademark.
Second, if you what you want is a name or word that identifies your products, but you sign your contracts using your legal name, you probably want a trademark. A trademark is a symbol, a word, sound or any other "tool" you use to identify the commercial source of a product or a service in the market and to distinguish it from others that can be similar. To have a trademark you must use the name (the word) as a trademark, i.e., you must mark the goods or services. The use of a trademark, as a trademark, is what gives you rights. Thus, in the case of goods the use of a trademark in receipts or publicity is not enough, you must mark the product with the mark. In the case of services, the use of the trademark in marketing material works. Finally, keep in mind that you can search any name in the website of the Secretary of State or the USPTO to assure availability and avoid confusion or deceit.
Finally, if you want a domain name, remember that it cannot be used to confuse consumers or to dilute the distinctiveness of an existing trademark. Then, you should avoid something like "googleit.com" or "nikefun.com", something like that, when famous trademark are used to register domain names. This is called cyber-squatting and it can create many legal problems.
As a final though, to start, you don't need to register anything, at the beginning entrepreneurs test many options. Then, once an idea has become into a real project, they consider things like the registration of a business name, a trademark or a domain name.