You can legally start using your brand name and logo after applying for trademark registration, but it's not without risk. While your application is pending, you have no active legal protection. If another company already owns a similar registered trademark, you could face infringement claims, including cease and desist letters, forced rebranding, and potential lawsuits. It's advisable to conduct a thorough trademark clearance search or consult an intellectual property attorney before launching your brand to mitigate these risks.