What is the main idea of Taoism?

Is it saying that people shouldn't think?

Nah, it's a nice philosophy for its day. It's outdated now.

No, its saying that you should go with with the natural way of things, not against them.

Taoism is all about going the natural way, literally. Staying with nature. Tao literally translates to "way" or "path" and is used in both Taoism and Confucianism (as they both kind of go hand in hand, although they are not the same). It is to signify the fundamental or true nature of the world.
It doesn't say people shouldn't think, just that people should think along the straight path of nature.

Work without effort.

It really says guard against being too clever rather than prohibiting thought.