First of all, hopefully you've already filed for a trademark with the US Patent and Trademark office. (You may also have a common law trademark or a state trademark).
If your trademark has already been approved you can begin by sending a cease and desist letter to the person infringing.
If that doesn't work you can also hire a lawyer to file a claim with the US Patent and Trademark office. Often you will need to act within the first thirty days. You will have to prove:
- that you have a prior claim on the mark
- the infringement might cause customers to confuse the other person's product with yours.