If you are filing a claim for PTSD, you need to complete VA form 21-0781 to submit with your application for VA compensation. This form requires detailed information about the event(s) that you believe led to your PTSD. You should give as much information as you can remember. Many times the VA will need to verify your stressful event using military records. If your PTSD is a result of a personal assault, you should complete VA form 21-0781a rather than the standard 0781 form.
If it will take you some time to be ready to file your VA claim, consider filing an ITF form.