Formal:
* 我中文不太好。 (Wǒ zhōngwén bù tài hǎo.) - This literally means "My Chinese is not very good."
* 我的中文水平有限。 (Wǒ de zhōngwén shuǐpíng yǒuxiàn.) - This means "My Chinese level is limited."
Informal:
* 我中文不太行。 (Wǒ zhōngwén bù tài xíng.) - This means "My Chinese is not very good," in a more informal way.
* 我中文很烂。 (Wǒ zhōngwén hěn làn.) - This means "My Chinese is very bad" and is very informal and blunt.
* 我只会说一点中文。 (Wǒ zhǐ huì shuō yīdiǎn zhōngwén.) - This means "I can only speak a little Chinese."
To express that you're learning:
* 我正在学习中文。 (Wǒ zhèngzài xuéxí zhōngwén.) - This means "I am learning Chinese."
* 我正在学中文,还没学好。 (Wǒ zhèngzài xué zhōngwén, hái méi xué hǎo.) - This means "I'm learning Chinese, but I haven't learned it well yet."
Remember to use the appropriate level of formality based on who you are talking to.