With basal body temperature charting, you're looking for an overall pattern, as opposed to a temperature spike here or there. Your temperature may rise and fall as your cycle progresses, but you should notice a biphasic pattern after ovulation. This means that before ovulation, the temperatures are on average lower than they are after ovulation.
For example, in the photo above, the day the temperature dips is the day ovulation occurred. We know this because the temperatures after this day are relatively higher than they were earlier in the cycle.
After you see at least three higher-than-average temperatures in a row, you can most likely say that ovulation occurred on the day before the first high temperature. If you've been tracking your cervical mucus, then you can be even more sure ovulation occurred on the day before if you noticed fertile cervical mucus on the days leading up to the temperature rise.
If you're lucky, you may notice a strong dip in temperature on the day of ovulation. Not every woman gets this nice heads up, but if you do notice a consistent dip in temperature before the rise from month to month, you should be sure to have sexual intercourse on that day.
Some women attempt to detect early pregnancy using their BBT charts by looking for implantation dips or triphasic chart patterns. Unfortunately, having (or not having) these patterns can't tell you if you're pregnant or not.