LOL - RIBS, a very valid question! You need a (I think it's called) Basl Thermometer so it should give a reading with 36.77 or something (so two digits after the decimal point) because the changes in temp can be really slight.
Have a look on Ebay (that's where I got mine last time). It can be an 'internal' reading and that's what some people prefer, personally I stuck it under my tongue. But either works!
You have to keep it there for 60 seconds and wait for the beep before you take it - believe me, the days after OVing that 60 seconds will be the longester ever!!
If you are PG, the temp will stay up after OVing, if not, it will start to drop.
However DISCLAIMER don't rely just on this and think that you're not PG if it drops, because other things can of course effect your body temp.
ALSO you cannot do ANYTHING before taking your Temp and it MUST be first thing in the morning. You can't even get out of your bed, that's how sensitive our body temp is. Take it the minute you wake up, so keep it within arms reach of your bed. I've heard that even talking to your OH before taking it can effect it.
I just used to turn off the alarm, take the temp, make a note of it and then get up. If you want to start charting temps and symptoms, try something like Fertility Friend.
All sounds a bit much I know, but for someone like me who's very organised and needs to have things to focus on, it was something to keep me going.
DH didn't know anything about it by the way, I think some men freak at the clinical nature of TTCing!! LOL
HTH hun. xx