As you haven't mentioned any particular exam, I'm sharing a few steps which might help.

For any exam, if the application form is released, you either have to fill it online and submit it or you have the option to download it. There will be a download button along the form. Click on that and your form will get downloaded to your computer. Open it and start filling up the details asked like name, father's name, your qualification (10th, 12th or any other) etc. You can then send the form along with any other required document to the address mentioned for that particular exam.

Can you please tell me you are asking for which exam?