Yes it is true that you need to make an appointment for a interview with Social Security after you filled out your application.
I just have a few questions that if you could please answer would help me in answering your questions better.
First of all it can be allot harder to get approved for SSI or SSDI when you are only 24. That doesn't mean it isn't impossible, just allot, allot more difficult.
Are you seeing you a doctor right now and is she/he aware that you have applied for social security? It is extremely important that you have your doctor's backing and their documentation to show your disability. Without this it will be impossible to be approved.
Are you working now? And how much have you worked. The answers for these questions depends on what type of disability you will be approved for if you are. SSDI is usually for people that have worked allot. You get points (I think that is what they are called) for how many years you have worked. Now SSI is usually for people that have a very low or no income. There is a set amount of how much money that you can't have in the bank and also a set amount of money that you cannot be making a year. I do not remember what that amount is off hand.
Usually the first time around you will be denied. And I am guessing here, but I think because of your age you will be denied the first time. It is then usually the time that you can get a lawyer to help you. You could also right now have a lawyer take a look at your case and see what they have to say about your chances to get approved. They don't usually charge you for this service (I believe).
The main thing that you need to be sure of is that to make sure that you have your doctor's backing on this. Do they also believe that you are unable to work at any type of job? Because like I mentioned earlier without their help you really do not have much of a case, especially with your age.
I hope that I am not depressing you!!

But it is important that you know what you will be up against during this process. Some people have waited years to be approved. I was just recently approved. But it was my doctor that suggested that I apply for social security disability. I have reflex sympathetic dystrophy, it is a nerve disease, to put it in layman's terms. Anyway, I am 49 years old and worked at my job 22 years. But I had always worked at some full time job since the age of 16. I have been off of work because of the RSD since November of 2005 and I was approved October 9th of 2008. So that was three years since I had to stop working until I was approved. See I can still do math
I wish you all the luck in the world in getting approved. Perhaps if you could answer some of the questions that I asked we could help you better.
Take Care,
Chris