If you’ve decided you need to trademark and consulted with a trademark lawyer, you should start by doing a search for existing trademarks. You can use the US Trademark Electronic Search System to search.
If things look good, it’s worth having a professional do a more exhaustive search to make sure there are no potential conflicts. From there, an application needs to be prepared and filed.
Applying is complicated, so I recommend paying a lawyer or using an online service like LegalZoom to make sure it’s done correctly.