how should I use at, examples: you are goot at writing, is it just when someone is really good, ?
Or can I say you are bad at writing?

Question

how should I use at, examples:  you are goot at writing, is it just when someone is really good, ?
Or can I say you are bad at writing?

Answer

It doesn't mean "really good" just "good".

While you can use this construction of "good at -ing" I think it sounds smoother to use the noun form of the action verb.

You are a good dancer.
You are a good writer.

You are a bad singer.
Etc.

Answer

Yes, you can say both of those - 'good at writing' or 'bad at writing'.

You can also say good/bad at for other things that people do, for example,  'good at dancing' or 'bad at singing'.