hey ppls!!!

i have a question about my thermometer, i have started reading the charting lessons(which seems to be so roundabout and confusing) and i get the bit where you have to take your temp everyday before you get out of bed, but i have a cheap $20 thermometer and dont know if its ok to use?

i takes temp in your mouth is this right for a BBT temp or do i have to go buy another one?
i threw out the packaging and cant remember what it said....

lisa