我今天工作了

It is verbose to mention 工作 twice.

我工作的得很快可是我不想在餐厅不想工作。

When you use structure like verb + adverb, you should use “得” to connect two parts.

我下明年在中国教英文，觉得 那太好了。

There's no expression like “下年”, we usually say "明年", and “下一年” is also ok but seems to be strange because it is too formal.

觉得 那太好了 I cannot understand this  sentence, sorry. If I you meant: Next year I will teach English in China, I think that's good. you should say “我觉得那很棒”。 "我" can not be omitted. and 太好了 in Chinese means "Oh, that's good", "Oh yes!", usually used on occasions like something bad may happen so you keep afraid of it but at last it doesn't happen. Just like, when you were declared this Saturday you should still go to work but after a while you are told that the work of Sat has been canceled, then you can say “太好了”.

我今天工作了

我今天工作了