He aquí una frase dominguera para decirlo: I'm not very conversant with the subject. O sea, "versado". Y también una frase informal para el resto de la semana: I'm not well up on the subject. Modismo: "to be up on" = estar puesto en

It means "I don't know a lot about it", for when you're talking about a certain topic such as global warming or American politics. (In contrast, "I'm not very good at this" would be "no soy muy bueno/a en esto", for when you're talking about a skill or school subject such as sewing or biology.)

Based on the context, I think it would mean something like "I'm not very good at this" because in English, it isn't typically said that "I am not very good at this subject." (Dory! Sigue nadando, sigue nadando xD Buscando a Nemo!) I hope I helped ^^