To convert an OBJ 3D model, use 3D software (e.g., Blender, Autodesk Maya) or online tools (e.g., Convertio) to import the OBJ file and export it to your target format (STL, FBX, GLB).
Choose tools by needs: Blender (free, full control) for custom work; online converters for quick, no-install jobs. Pick output formats based on use—STL for 3D printing, FBX for game engines (Unity/Unreal), GLB for web AR/VR (Three.js). Note: OBJ uses a separate .mtl file for materials—include it to keep textures.
For beginners, start with Blender (open-source) or online tools. Upload both OBJ and .mtl if textures matter, then test the output in your end app (e.g., 3D printer software for STL) to ensure compatibility.