The NLRA protects both your right to be a union member, and your right to choose not to be a union member. If you feel that you have been threatened for not joining a union, you must first file a charge with your local NLRB office. This is commonly referred to as an Unfair Labor Practice charge. Then the NLRB staff will conduct an investigation to see if your claim has merit - if the investigation leads to the conclusion that something wrong happened, then the NLRB will attempt to reach some sort of agreement to settle or fix the problem. If the NLRB decides that your claim does not have merit, then you may appeal that decision, but you may need the help of an attorney.
If you think you have been retaliated against for not joining a union, see our retaliation for union activity page for additional information