Formal:
* I am of half Chinese descent.
* My heritage is half Chinese.
* I am half Chinese and half [other ethnicity].
Informal:
* I'm half Chinese.
* I'm Chinese on my [mother/father's] side.
* I'm half Chinese, half [other ethnicity].
More specific:
* I am a Chinese-American/Canadian/etc. (If you have a specific nationality)
* I have Chinese ancestry.
It's also important to note that the way you identify yourself is personal. You might feel more comfortable saying one of the phrases above over another.
Remember, there's no right or wrong way to express your heritage. Choose the phrase that feels most natural and comfortable for you.