yellow part: Where I think not good enough
blue part: My comments to the yellow part
purple part: What I added to your work.
(If there are something wrong in my English, please let me konw :) )

你好!(For there is a "大家好" below, "你好" can be deleted.  "你" is a part of "大家")

大家好!

我的明之(名字)叫黄佩君(Good name!)...今年二十二岁.

我是学生. 我的专业是市场营销.

我觉得华文(although "华文" is ok, "中文" is much more widely used)有一点难学应为(因为)很难记得(记忆 is better here..."记得" seems not wrong, but it is wired here. "记得" always used to decorate something to do or some event, "记忆" should be used when you means remember knowledge in my opinion).

很想("希望" is better)有一天我会读中文.

希望多多指教!

Good job~Enjoy Chinese~

你好!

大家好!

我的名字叫黄佩君...今年二十二岁.

我是学生. 我的专业是市场营销.

我觉得中文有一点难学因为很难记得.

很想有一天我会读中文.

希望多多指教!