Contrary to popular belief, you do not need to register a trademark to have rights in the mark. Rights in a trademark arise in the United States from use of the mark. What is a trademark? A trademark is generally a word, phrase, symbol, or design, or a combination thereof, that identifies and distinguishes the source of the goods of one party from those of others. A service mark is the same as a trademark, except that it identifies and distinguishes the source of a service rather than goods. Marks typically protect brand names and logos used on goods and services.
However, you can be creative on the subject matter associated with a mark- even a smell or fragrance can be subject to trademark protection in some instances, for example the smell of Play-Doh (described as "a unique scent formed through the combination of a sweet, slightly musky, vanilla-like fragrance, with slight overtones of cherry, and the natural smell of a salted, wheat-based dough").
While you are not legally required register a mark, registration is normally a good idea as it confers a number of benefits. These include an expanded scopes of protection, a legal presumption of ownership (a good thing if you ever have a dispute over the mark), the ability to bring an action concerning the mark in federal court, and more.
Check out the USPTO's helpful resource on trademark basics if you'd like to know more.