Yes, so long as you did not sign a non-competition agreement and do not use any of their propriety material/information, their equipment or software, run the business on their time, or use their intellectual property, you may start your own business and compete with your employer: in the U.S.'s "employment at will," where employer and employee have very few obligations to each other, you can have your own business while employed or compete with your employer so long as you are not using their materials or property and did not contractually agree to not do this. Of course, if they become aware you are doing this, they could terminate you, so make sure you don't need your current job or are willing to lose it before you proceed.