It really is hard to say because there are many slots and we do not know what your teacher is teaching. I have seen a lot of teachers teaching out of date material. With that said, here is a list from best to worst.
PCI Express x16
PCI Express x1
AGP
PCI
This is list is based on greatest to least and base on the bandwidth read and write speed.
Answer:
Pay attention to what you buy and limit yourself to an amount of money everyday.
Explanation:
This is just a simple way from spending a lot and limits your everyday spendings when you actually leave an amount for yourself.