Basil body temps?

Amber • 19 been with my fiancé for two yrs. pregnant with our first baby! Due febuary 27,2015!!(:
How do you record your bbt? Where do you buy the stuff for it? How does it work?i am thinking about trying to do bbt but I have never done it before so I don't know where to get the stuff or how it works or anything someone please care to explain?? I would much appreciate it.