To open an obj file in Blender, follow these steps:

Step 1: Launch Blender and go to the 'File' menu at the top of the screen.

Step 2: Select 'Import' and then choose 'Wavefront (.obj)' from the list of available file types.

Step 3: Navigate to the location of your obj file on your computer and select it for import.

Step 4: In the import settings, you can adjust options such as scale, rotation, and materials if necessary.

Step 5: Once you are satisfied with the import settings, click 'Import' to bring the obj file into your Blender workspace.

That's it! You have successfully opened an obj file in Blender and can now begin working with it in your 3D modeling and design projects.