I am no intellectual property attorney, but it is my understanding that registering a trademark is not necessary to have the right to prevent others from later
beginning to use the trademark. However, registering makes it easier to prove the date from which you have rights to the trademark.
The process to register a trademark may be simpler and less expensive than you expect, if you will be satisfied with a state registration. It may be something you can do yourself for well under $100.
There is lots of good information from the US Patent and Trademark Office at http://www.uspto.gov/trademarks/index.jsp