举一反三

举一反三 (jǔ yī fǎn sān) 

It literally means to list one and infer three. When study, we learn by analogy and infer other things from one fact.

乱七八糟

乱七八糟 (luàn qī bā zāo)

It means that something is a total mess. 

一无所有

一无所有yìwúsuǒyǒu 

It means to have absolutely nothing at all. 

一见钟情

一见钟情 (yí jiàn zhōng qíng) means love at first sight. 

无风不起浪

无风不起浪 wúfēngbùqǐlàng

It simply means nothing happens without a reason, similar as the meaning of “No smoke without fire” in English.

人山人海

人山人海 (rén shān rén hǎi) 
This is commonly used to describe a lot of people, or a very crowded situation.