The right response depends on the nature and location of the infringement.
If a similar trademark application has been filed in a jurisdiction where you hold a registered mark, file an opposition during the publication window. This is the most time-sensitive action; missing the window may be irreversible.
If someone is infringing on your mark on a social media app, app store, or e-commerce marketplace, then submit a platform IP complaint or takedown request. Most platforms provide a dedicated form for this purpose.
If a confusingly similar domain has been registered, consider a UDRP complaint through WIPO or a direct approach to the domain holder.
For commercial infringement in your territory, a cease and desist letter is the standard first step, escalating to legal proceedings if necessary.