'Get you a notebook' is not correct English.
If you are in class and you don't have a notebook, your teacher might say 'Get a notebook' or even 'Get yourself a notebook'. This means that you should go and find a notebook - for example, you should go to a stationery shop, buy a notebook, and bring it to the next lesson.
The word 'get' has many different meanings in English, but this example 'Get a notebook' is one of the most important meanings. It means 'obtain'.
NB 'Get' in the sense of 'understand', as in 'I get you', is an idiomatic and colloquial use. It is much less important than the meaning above.