The very first thing you should do if your veterans’ disability claim is denied is contact the VA immediately and tell them you disagree with the decision by putting it in writing. The VA calls it a “Notice of Disagreement” or NOD. You only have to send a short note or letter saying you disagree with the decision and that you want to appeal. You must say you want to appeal.
You can also hire legal representation to help you with appealing your denial.