Trademark ownership gives you the legal ground for fighting infringement, but the burden of enforcing those rights falls upon you. Therefore, if you discover a business that copies your brand, you have two options - you can contact them personally or hire an attorney to contact them on your behalf.
Before reaching out, you should make sure your trademark protection applies. Double-check if the brand in question operates in a jurisdiction where you hold your rights. For example, if you find a brand using a very similar name in the UK, but you are protected in the US, you cannot stop the third party from using the name.
Next, check whether their offering falls under one of the classes in which you registered your trademark. For example, if you own a trademark in Class 25 (Clothing) and the other business operates a taxi service, your protection won't apply.
If you believe you have full rights to the brand name and the other party refuses to make any adjustments, you can hire a lawyer to send a cease and desist letter on your behalf.