Start taking it now, and if you know your cycle day, just mark it down. I believe the ovulation tracker on this site has a place to put your temp, and even graph it nicely, it's also easy to ask others on this site for imput on what it all means! One number won't be too much, you'll need a month's worth to start getting an idea.
I was using basal temp to time my pregnancy.
so every morning at the same time (set your alarm), before you do ANYTHING! take your temp and record it..
when you get your period.. mark an X on that date, at the temp on the chart.. when your period ends, put dots at the temp you get each AM.
you will notice that for teh first week or so, your temp will bounce very little..
when you see your temp increase .4 degrees F or so.. and its increasing more and more for 3 days straight.. that means your ovulating!.. that will last for only a few days.. if your temp goes back down to where it was before ovulation, your PMSing.. and soon you'll get your period..
BUT.. if your temp stays high for a long period of time (they say 18 days).. and dont get a period.. YOUR PREGNANT!
.. hope this helps. and best of luck!
You can start taking your temp any time during your cycle! You probably won't see much of a pattern for a month or so.