楼上的句子到处都是语法错误.要改你的句子都要写一大篇了.干脆只告诉你正确的句子吧：
It was Lily who came up with the idea that people should drink more soft drink.
这里,idea和that people should drink more soft drink是等同的,所以that people should drink more soft drink是个同位语从句.注意idea不是楼上所说的主语.这句话的主语是It.
略微把上句改一下,就可以造出一个主语的同位语来：
Lily, the girl who came up with the idea that people should drink more soft drink, is in my class.
这样,the girl就是Lily的同位语.后面的who came up with the idea是定语从句.而that people should drink more soft drink仍然是idea的同位语.
记住同位语和它所修饰的词是完全等价的.再给你一些例子：
My sister Helen will come with us.
Helen是同位语,修饰my sister.
The news that he died shocked everyone.
that he died是同位语从句,修饰the news.