You can express it in different ways in English, like; Your are at home with mathematics. You are well acquainted in linguistics. You are expert in Chemistry. You are proficient in Physics. You are adept in IT. and so on and so on.....

I'll add "well versed" to the list. It sounds like you know it poetically, even if it is something like accountancy. ;) For your example, you could say "I have a strong background in IT", or "I have a good knowledge of IT". I'd use the first phrase, as it tells me you have experience as well as knowledge.

I am in expert in "something".