You are going to have appeal the descion. The letter should tell you how to do that. Write to them straight away to tell them. You will probably have to go to have a tribunal hearing. It will take a quite some time for the appeal to be heard. You should be entitiled to income support during this time.
Then start collecting evidence, your GP or any other medical proffesionals you see can write you a letter stating how your condition affects your daily life.
Also it useful to keep a diary of how you feel and how your condition is affecting you on a daily basis.
I helped one of my clients appeal against an Incapacity benefit decision. HE won, the diary influenced their decision because they could see that his condition prevented him from holding down a job.
ALso go to CAB and get advice, they will be able to help you prepare a case.