To reduce a 3D model with texture atlases as a beginner, focus on simplifying geometry and optimizing the texture atlas itself.
Use Blender’s "Decimate Modifier" (a beginner-friendly tool) to lower polygon count—start with a low ratio (e.g., 0.5) to keep key shapes. For the texture atlas, resize textures to a smaller resolution (e.g., 2048x2048 instead of 4096x4096) or use compression like PNG (for transparency) or moderate JPEG (for photos) to cut file size without too much quality loss.
Test the model in your target app (e.g., Unity, Sketchfab) after small changes—this helps balance file size with how the model looks and performs.