Use 個 if the correct measure word is unknown
You should learn those measure word one by one because most of native speakers don't use "個" for every nouns. But sometimes if you forget or don't know the word you should use, most people still know what you're referring to with the word "個" in most case.

Why I said in most case? Here's an example: (Actually, it's a joke.)

The foreigners usually have a big trouble if they should use "個" and "子". "個" is a measure word(量詞) when you have no idea which word you should use while "子" is something we add after a thing for better reading, like "桌子", "椅子".

So, one day. Tom walked into a watch store and ask for a new watch: "老闆，我想要買一個錶子。"(Lao ban, wo xiang yao mai yi ge biao zi).