I'm worried it might rain. /I'm afraid that it will rain. 

我担心会下雨。

I'm afraid to make mistakes.

我担心会犯错。

I'm afraid Mom won't get better soon.

我担心妈妈不会很快好起来。

I'm worried that I mess up the exam.

我担心开始会考砸。

I'm worried because my best friend wouldn't talk to me.

我担心，因为我最好的朋友不再跟我说话。

Don't worry too much. /Don't be too anxious.

不要担心太多。

You don't need to worry.

你不需要担心。