This is one of the most frequent mistakes I see with English. No-one is "good in" something. The collocation is, "to be good AT (something)". Same as if one says "interested in..", "fascinated about...", "keen on..." and so on. These are the collocations - don't think they can just be swapped around. I worry that this mistake is made so often that people might think it is acceptable English. If someone tells you, "I'm good in English", you can be pretty certain that they are not! ;)

You should use "I'm not good at English". In can be used in certain sentences where a noun is used instead of a verb or adjective. Example: "I have a degree in English" or "I have a black belt in Karate"

"at" is better than "in". We tend to use "good at" for skills: I'm not good at football, I'm not good at cooking, I'm not good at speaking Russian.