What determines disability?

I have epilepsy I take very expensive prescription medication monthly as well as antidepressants. I was informed that I could file for disability. I am 21 work part am able to drive as long as I continue my medication. It is hard to make it without my parents help, they already pay my Insurance but is this something that could be a detriment to me in the future? Do I have to go on Medi Cal?