The day before or day of your period your temperature drops and stays low (or may even drop further after af leaves) until you ovulate. The day after you ovulate, your temperature spikes up to a higher level and then stays high. Once your temperature spikes, it is normally too late to conceive, so it's best to chart your cervical fluid and/or use opk (ovulation predictor kits) as well in order to catch the eggie.
I highly recommend the book "Taking Charge of Your Fertility." It explains how all of this stuff works. Your local library should have it.
GL!