I have a m/c about 8 months ago an i been trying to get pregnant but still haven't i have 2 sons already so i know what im doing something wrong?

Give it more time: The odds of getting pregnant any one cycle is only about 20%. Factor in the stress of the recent miscarriage, and the lack of energy/privacy that comes with having 2 kids at home, and this may be totally normal. You're still very young. If you're having regular periods, then wait until it has been a full year before getting worried. If your periods are not regular, however, see a doctor sooner.