To qualify for disability benefits (either SSDI or SSI), you must prove you have a severe, medically determinable impairment that limits your functioning to such an extent that you aren't able to work. Disablity Determination Services (DDS, a state agency that evaluates disability claims for the SSA) analyzes each disability claim with a five-step evaluation process.
Here are some tips on getting your disability claim approved, from disability lawyers who have helped many claimants get benefits.