Being, a cancer and a hepatitis patient.and im underweight , can i go to gym for little workout and to gain weight.is there any problem ?

Absolutely: Check with your physician, but i'll bet that he/she will encourage you strongly. I am extremely glad that you want to do this. The common bone cancers at your age are ewing's and osteosarcoma; both are very often cured nowadays. Hepatitis b can usually be managed, though not cured. Hopefully you will be a long-term cancer survivor turned adult lifetime athlete. Best wishes.