DIY: add freon

I got a used civic last summer, the AC was working well then.  But now it is not blowing cool air.  I am thinking of adding some freon to it by myself.  

I called Autozone, they sell a set of things for 30 dollars:  a small can of freon, the gauge and hose.  They also sell things separately, like freon, hose, at different prices. 

If i buy the whole set for 30 dollars, do i need to buy another can of freon?  What are some of the tricks I need to be careful if I add the freon by myself?

Thannks a lot!