If the VA Regional Office says your disability is not service-connected or if the percentage of disability is lower than what you think is fair, you have the right to appeal to the Board of Veterans' Appeals. The first step in appealing is to send the VA Regional Office a "Notice of Disagreement" This Notice of Disagreement is a written statement saying that you "disagree" with the denial. Be sure your Notice includes the date of the VA's denial letter and be sure to list the benefits you are still seeking.
In response to the Notice of Disagreement, you will get a "Statement of the Case" from the VA Regional Office. This will repeat the reasons stated in the VA's denial letter why your claim was denied and will include the relevant VA regulations. Once you get the Statement of the Case, if you still wish to pursue your appeal, you should file a VA Form 9, "Appeal to Board Veterans' Appeals" which is sent to with the Statement of the Case. You have 60 days from the date on the Statement of the Case, or one year from the date the VA first denied your claim, to file the VA Form 9. Whichever date is later is your deadline.