Here's a breakdown:
* Мой (moy) - my
* Русский (russkiy) - Russian (language)
* Плохой (plohoy) - bad
You can also use these phrases, depending on the context:
* Я плохо говорю по-русски. (Ya ploho govoryu po-russki.) - I speak Russian poorly.
* Я не очень хорошо знаю русский язык. (Ya ne ochen' khorosho znayu russkiy yazyk.) - I don't know Russian very well.
* Мой русский язык не очень хороший. (Moy russkiy yazyk ne ochen' khoroshiy.) - My Russian is not very good.
Remember to be polite and humble when talking about your language skills.