If my head CT scan radiation gonna cause a brain cancer to me I need to start worry about it after 5 years or 20 years and more after the scan? Scared

Very unlikely. If you are asking about the risk after a single head ct scan it is not measureable and is not considered a risk for developing a brain tumor. If for some reason you have many routine head ct scans especially if done throughout childhood there is a very small slight increase risk of developing a tumor but far less than .5%. The low dose of radiation from modern ct scans is negligible if used safe.
SMALL risk. The risk is delayed by about 15 to 20 years. But even after 20 years, the risk is extremely small. Obviously, we don't want to get cts for no reason, but if you need one, i would not worry about a tumor afterwards.
Don't worry. Though there is a very small increased risk of certain cancers associated with radiation, the risk is much higher in young children who have multiple ct scans. Brain tumors are rare and the absolute risk for an adult who has had one or a few head cts is only very slightly higher than those who do not have head ct scans. The risk is still very low and should not cause you any anxiety or fear.