Why I come* here.

Hey, I come* to italki to look for a good partner to help me to improve my English level (you don't have to say 'level'). I come from mainland China and live in Guangzhou city (you don't have to say 'city'), so I can speak Chinese and Cantonese. Maybe I can help you improve your Chinese [level] if you want to learn how to fluently speak a fluent and real Chinese. I hope we can help and encourage each other to improve our language ability/abilities. I also hope we can build up a very nice friendship.

*Just to make sure that you mean 'I come' instead of 'I came':

I come & I'm coming (present), I have come (past participle), I came (simple past).

What you said is fine, but just remember it's in present tense.

Why I am here

Hey, I came here on  italki  looking  for a good partner to help me  improve my level of  English.I am  from mainland China and live in Guangzhou city, so I can speak  both Chinese and Cantonese. Maybe I can help you improve your Chinese level if you want to learn how to speak a fluent and real Chinese. I hope we can help and encourage each other to improve our language ability. I also hope we can build up a very nice friendship.