What can i do if someone has a very similar brand name to mine? the difference is that i have it trademarked and they don't.

Get a trademark lawyer quickly. You may have prior common law rights in your mark depending on how and for how long you have used that mark. You may be able to assert those rights against a subsequent user of a confusingly similar mark. Still, don't charge in thinking that this is a potential windfall: trademark infringement cases can be expensive and you can get lost in them. Get a lawyer who will listen carefully to your issue and who will give you several options, including a peaceful settlement, that are more in line with your business goals.

First evaluate with an experienced trademark/business attorney what your real goals are. Moving quickly is good, but not without reflection and direction. A cease and desist letter is normally the first step. Considering what tone and what relief you request in the original letter, and further considering what you will do in either alternative response (OK or get lost), or no response, is where the value of an attorney is to be found.

Obtaining trademark registration does not guaranty that you would prevail in a dispute with someone who did not do so. Trademark rights generally arise from use in commerce, not registration. What matters is which of the two brands is the "senior user"----i.e., who started using it first. If the unregistered brand started using it first, it could sue you for trademark infringement even though you registered. You need to retain intellectual property counsel to investigate the facts and circumstances of your case---and you should do nothing on your own until a determination is made as to whether you are the senior user or a junior user. In a situation like this, it often makes sense for the two trademark owners to attempt to settle the matter by cross-licensing each other. Perhaps each company can agree to use the similar name in the context of a distinctive logo or artwork so that the two companies could be more easily distinguished from each other in the eyes of consumers. Trademark litigation is expensive, time consuming and often unsuccessful. It is always better to try to settle these matters in a situation like this. Also, note that if you send a cease and desist letter to this other company before you determine whether you are the senior user, you could be inducing litigation that you don't want and can't afford. Be careful here---don't do anything without legal counsel.