If your application for disability has been denied and you intend to appeal, you can either file a reconsideration appeal on your own, or find a claims representative to do this for you.
In all likelihood, if your first application failed, your reconsideration appeal will be unsuccessful as well. A representative (a disability attorney or non-attorney representative) may be able to help change your odds if you omitted something important on your application, but there's no guarantee. This means you will likely have to file a second appeal. The second appeal is decided by a federal administrative law judge (ALJ) at a disability hearing, and before your hearing you should absolutely have legal counsel. (Here's how a lawyer will help you at the hearing.)
Study after study has shown that ALJs are significantly more likely to award disability benefits to those who are represented than to those who choose to represent themselves. Typically, about 40% of unrepresented individuals who apply for SSI or SSDI and have their case taken before a administrative law judge (ALJ) will end up winning disability benefits. About 60% of individuals who are represented (by an attorney or non-atttorney rep) will end up winning disability benefits.
That said, your reconsideration is likely to get denied, in which case you'll need to hire an attorney for the appeal hearing anyway, so unless you your medical condition has gotten significantly worse and you have a good chance of getting approved at this stage, you may want to hire an attorney now.
Most people are capable of filing their own appeal paperwork, and there is now the option of filing online, which makes it even more convenient. However, there is the tendency among those who file reconsideration appeals on their own behalf to procrastinate, to the point of missing the deadline. Social Security allows 60 days from the date of denial (plus 5 days grace for mailing time) for an appeal to be at the Social Security office.
A surprising number of disability claimants miss this deadline; they make the mistake of thinking the deadline is that their request for reconsideration has to be postmarked within 60 days from when they receive their notice of denial. It actually has to be in the Social Security office, not postmarked, within 65 days from the date stamped in the upper right corner of the envelope that contained their denial notice. This is one reason why it can be a good idea to get a lawyer before you file an appeal--a good disability lawyer will file all appeal paperwork for you on time.
If you do decide to hire a disability attorney or non-attorney rep for your reconsideration appeal, or for a disability hearing, don't put this task off until it's too late for the lawyer to be of any real help to you. In order to act on your behalf, your lawyer or rep must have certain forms signed, such as a fee agreement, permission to get your medical records (Form SSA-827, which is a medical records release form), and Form SSA-1696, which officially designates your chosen lawyer or rep as your representative. If you wait too long to hire an attorney, there will not be enough time to complete the forms, and you may end up having to request an appeal on your own anyway so that you don't miss the 60-day deadline.
Learn more about the Social Security disability appeals process.